Method of controlling hydraulic pressure in speed change mechanism having hydraulic clutch

ABSTRACT

A speed change mechanism ( 1 ) constructed by connecting in tandem a hydraulic type speed change unit ( 17 ) having a plurality of hydraulic clutches ( 57, 58, 59 ) to be alternatively engaged and a hydraulic type speed change unit ( 20 ) having a plurality of hydraulic clutches ( 66, 67, 68 ) to be alternatively engaged, wherein a time-varying region (common slip region) is secured in which the two clutches slip in common such that during speed change, when the working hydraulic pressure in a clutch to be engaged is on its way to gradual increase, the working hydraulic pressure in a clutch to be disengaged lowers. This common slip region is made smaller during shift-down than during shift-up by a change in time-difference between the pressure increase start time for the clutch to be engaged and the pressure decrease start time for the clutch to be disengaged or by a change in the pressure decrease property of the clutch to be disengaged, and is maintained constant irrespective of whether one or two hydraulic clutches are engaged and disengaged during speed change respectively or irrespective of a difference in engine rpm.

TECHNICAL FIELD

[0001] The present invention relates to a method of controllinghydraulic pressures in a speed changing mechanism having a plurality ofhydraulic clutches, that is, a hydraulic power shift speed changemechanism. Particularly, the invention relates to a method ofcontrolling hydraulic pressures in a multistep-speed-type speed changemechanism constituted such that a plurality of hydraulic type speedchange units are connected in tandem, wherein each of the hydraulic typespeed change units is constituted of a plurality of transmission trains,and a hydraulic clutch is provided in each of the transmission trains.

BACKGROUND ART

[0002] Conventionally, there is publicly known a so-called hydraulicpower shift speed change mechanism configured of a plurality ofhydraulic clutches (fluid-operated multidisc clutches). Particularly,there is publicly known a multistep-speed-change-type speed changemechanism constituted such that a plurality of hydraulic type speedchange units are connected in tandem, wherein each of the hydraulic typespeed change units is constituted of a plurality of transmission trains,and a hydraulic clutch is provided in each of the transmission trains.In vehicles including the speed change mechanism, such as anagricultural and other work tractors, speed-changing for the number ofsteps obtained by multiplying the numbers of transmission trainsprovided in individual speed change units. Suppose a speed changemechanism configured of two hydraulic type speed change units, in whichtwo transmission trains are provided in one of the hydraulic type speedchange units, and three transmission trains are provided in the otherhydraulic type speed change unit. In this case, 2×3 steps are obtained;that is, totally, six-step speed changes can be performed.

[0003] Conventionally, to perform input/output control forengagement/disengagement operating fluid for individual hydraulicclutches in the above-described speed change mechanism,electromagnetic-type selector valves are used.

[0004] In connection with the conventional hydraulic-pressure controlfor the hydraulic clutches at the time of speed-changing, first of all,the relationship in time between engagement-objective clutches anddisengagement-objective clutches will be described below. Essentialthings regarding speed-changing include the prevention from a case wheredouble transmission trains are operated to be in transmission states.Specifically, in the above-described multistep-speed-change-type speedchange mechanism configured by combining the plurality of hydraulic typespeed change units, it is essential to avoid a case where two clutchesare operated in an engaged state in each of the speed change units.Therefore, conventionally, a disengagement-objective clutch is firstdisengaged substantially completely; and after a nontransmission stateis once made in the speed change mechanism, the engagement of theengagement-objective clutch is then started. However, a high load isimposed during a nontransmission state, the vehicle is likely stopped.In addition, since a hydraulic pressure begins to rise from thenontransmission state when the engagement-objective clutch startsengagement, there remain problems which cannot be solved in that greatshocks occur, thereby causing an operator to feel uncomfortable.

[0005] In view of the above, as described below in the “Disclosure ofInvention” and in other portions, even when the transmission efficiencyis reduced to the lowest level during speed-changing, at least eitherthe disengagement-objective clutches or the engagement-objectiveclutches are controlled to be in slip states. Specifically, operatingtiming and a time-transitional hydraulic pressure property for theindividual disengagement-objective clutch and the individualengagement-objective clutch are set so that a region representing a slipstate (the region will hereinbelow be referred to as a “common slipregion”) for the two clutches can be secured.

[0006] Hereinbelow, a brief description will be made regarding clutchhydraulic pressures in the slip state. The pressure for a disengagedclutch in a fluid chamber is substantially 0, and a piston for operatinga clutch disc is in a free state. To engage the disengaged clutch,first, fluid is fed to a fluid chamber therefor to be filled out, andthe filled out fluid must be used to increase the pressure to hold thepiston. When a hydraulic pressure having a value that is sufficientlyhigh to hold at least the piston is set to a piston-holding pressure,the hydraulic piston is brought to a slip state at an operatinghydraulic pressure that is higher than the piston-holding pressure.

[0007] However, different from the above-described conventionalhydraulic-pressure control for which the relationship between theindividual hydraulic pressure states for the disengagement-objectiveclutch and the engagement-objective clutch need not be taken intoaccount, in the hydraulic-pressure control of the present invention,when the individual time-transitional hydraulic pressure properties forthe engagement-objective clutch and the disengagement-objective clutchare fixed as have been set under specific conditions where, for example,the engine is operated at a rated revolution frequency, there occurscases wherein no common slip region can be secured because of theconditional variations.

[0008] For example, in a speed change mechanism configured of twohydraulic type speed change units, there are two speed-changes. One ofthe speed changes is performed such that in one of the hydraulic typespeed change units, clutches remain held in engaged states; and in theother hydraulic type speed change unit, one engaged clutch isdisengaged, and a different clutch is newly engaged (one-objective-basedhydraulic clutches are disengaged/engaged). The other speed change isperformed such that, in each of the hydraulic type speed change units,one engaged clutch is disengaged, and a different clutch is newlyengaged; that is, in the overall speed change mechanism, totally, twoclutches are disengaged, and two clutches are engaged(two-objective-based hydraulic clutches are disengaged/engaged). Asdescribed above, before an engagement-objective clutch is controlled tobe in a slip state, wait time is required until the pressure increasesup to the level of the piston-holding pressure after the fluid isinjected into the fluid chamber of the clutch. For two-objective-basedhydraulic clutches to be disengaged/engaged, aforementioned time isrequired substantially twice as much as that in the case whereone-objective-based hydraulic clutches are disengaged/engaged.Therefore, when clutch-timing and a time-transitional hydraulic pressureproperty are set to secure a common slip region according to the casewhere the one-objective-based hydraulic clutches are disengaged/engaged,they are not suitable to the case where the two-objective-basedhydraulic clutches are disengaged/engaged.

[0009] When the engine revolution frequency is reduced, time requiredfor filling out the fluid in the clutch fluid chamber is increased.Therefore, for example, hydraulic-pressure control is set to obtain acommon slip region during a rated revolution. However, problems similarto the above can arise during idle revolution.

[0010] In comparison between a speed-changing operation at a shifting-uptime and a speed-changing operation at a shifting-down time, in theformer case, since the relative revolution speed of a secondary-siderotation shaft with respect to that on a primary side of anengaged/disengaged is increased, a common-slip-region period needs to beset to be relatively long. On the other hand, in the latter case, thespeed-changing is performed to reduce the relative revolution speed ofthe same secondary-side rotation shaft, and rotational inertia at a timeof preshift operation is imposed on the secondary-side rotation shaft.Therefore, the common-slip-region period may be short; and when it islong, smooth speed-changing is impaired.

[0011] As in the conventional case, in speed-changing in which anengagement-objective clutch is engaged after a disengagement-objectiveclutch is disengaged, detection is performed by using a pressure sensoror the like for the state of engagement of the disengagement-objectiveclutch that is supposed to have been engaged. Checking is therebyperformed for abnormality (such as entrance of foreign substances).Thereafter, engagement of the engagement-objective clutch isinterrupted, thereby allowing double transmission to be avoided. As inthe case of the present invention, in the speed-changing in which acommon slip region is secured, disengaging operations and engagingoperations of clutches are overlapped. Therefore, there can be caused acase where a disengagement-objective clutch is not disengaged, while anengagement-objective clutch is engaged. That is, there can be causeddouble transmission that can cause damage in the transmission mechanism.Therefore, an abnormality-detecting method suitable to the presentinvention is demanded.

[0012] Pressure-increase properties required for theengagement-objective clutches at the time of speed-changing aredifferent depending on the traveling mode of a work vehicle employingthe speed change mechanism; that is, the properties differ depending onwhether the vehicle is engaged in normal (on-the-road) traveling ortractional traveling. In a tractional travel time, the hydraulicpressure at a rising time needs to be set high, and the clutch needs tobe quickly engaged. Otherwise, the transmission efficiency is notsufficient to catch up with the load, thereby causing engine failure. Toreduce shock that can be caused in a normal travel time, rising pressureis preferably controlled as low as possible.

[0013] Conventionally, to overcome these problems, in ahydraulic-pressure control system for hydraulic clutches, two types ofpressure-increase properties, one for normal traveling and another fortractional traveling so as to be alternatively selected by an operatorare stored.

[0014] However, problems still remain pending resolution. With a controlmethod that is dependent on operator's switching operation, whenerroneous operation is performed, there occurs hydraulic-pressureincrease that does not correspond to practical requirements, causingproblems such as engine failure and shock generation. To cope with theseproblems, the control is preferably arranged such that the load state isautomatically can be detected, and one of thehydraulic-pressure-increase properties can be selected according to theresult of the detection.

[0015] In addition, as described above, the variety of conditions variesthe requirements regarding, for example, hydraulic-clutchengagement/disengagement operations at the time of speed-changing, i.e.,the time-transitional hydraulic-pressure-increase properties forengagement-objective clutches, time-transitionalhydraulic-pressure-decrease properties, and the operational timing: Tocomply with these requirements, it is preferable that input/outputhydraulic pressures for clutches be controlled to be variable; that is,it is preferable that the capacity of an individual clutch-operatingvalve be variable.

DISCLOSURE OF THE INVENTION

[0016] The present invention relates to a speed change mechanism(so-called hydraulic power shift speed change mechanism) having aplurality of speed-changing hydraulic clutches, each of which is engagedaccording to hydraulic-pressure-increase effects and is disengagedaccording to hydraulic-pressure-decrease effects. A primary object ofthe invention is to avoid a nontransmission state that can occur at atime of speed-changing with the speed change mechanism.

[0017] To achieve the object, according to the present invention, at atime of speed-changing operation, an operating hydraulic pressure for aclutch to be engaged from a disengaged state is gradually increased in atime transition, and an operating hydraulic pressure for the clutch tobe disengaged from an engaged state is reduced during the gradualpressure increase. Preferably, during the speed-changing operation, anoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch is set to be later than anoperating-hydraulic-pressure-increase start time at which a fluidchamber of the engagement-objective clutch becomes full of fluid, andthe pressure thereof rises to a piston-holding pressure. Thereby, atime-transitional pressure region (common slip region) where anengagement-objective clutch and a disengagement-objective clutchcommonly slip at the time of speed-changing operation is secured.

[0018] Also, in connection with the aforementioned object, in order toallow the common slip region to be constantly secured at all timesregardless of various conditional variations, at least one of a timedifference between the operating-hydraulic-pressure-increase start timefor the engagement-objective clutch and theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch and a time-transitional decrease propertyin the operating pressure for the disengagement-objective clutch iscontrolled to vary corresponding to engine revolution frequencies.

[0019] In this case, the various conditions include engine revolutionfrequency. Corresponding to the property that a fluid-chamberfilling-out time for the engagement-objective clutch increases inproportion to reduction in the engine revolution frequency, when thetime difference is controlled to vary, the aforementioned timedifference is set longer in proportion to reduction in the enginerevolution frequency or in a case where the engine revolution frequencyis equal to or lower than a specific level so as to decrease slower inproportion to reduction in the engine revolution frequency or in a casewhere the engine revolution frequency is equal to or lower than aspecific level.

[0020] In the speed change mechanism (so-calledmultistep-speed-change-type speed change mechanism) configured byclassifying the aforementioned plurality of speed-changing hydraulicclutches to allocate them to a plurality of hydraulic type speed changeunits, the hydraulic clutches are alternatively engaged in each of thehydraulic type speed change unit to thereby form one speed step. In thisconfiguration, as described above, in order to secure thetime-transitional pressure region (common slip region) where theengagement-objective clutch and the disengagement-objective clutch atthe time of speed-changing commonly slip, when the hydraulic-pressurecontrol in which the operating hydraulic pressure for the clutch to beengaged from a disengaged state is gradually increased in the timetransition, and an operating hydraulic pressure for the clutch to bedisengaged from an engaged state is reduced during the gradual pressureincrease at the time of speed-changing is employed, the number ofclutches to be engaged/disengaged is included as one of theaforementioned various conditions. Therefore, when the time differenceis controlled to vary, the time difference is set relatively long at atime of speed-changing when the number of the clutches to beengaged/disengaged is large, and the time-transitional decrease propertyis controlled to vary, the time-transitional decrease property is set tobe reduced slower at a time of speed-changing when the number of theclutches to be engaged/disengaged is large.

[0021] Considering that a rotational inertia is imposed at a time ofshifting-down operation compared to a case at a time of the shifting-upoperation, in order to reduce the area of a common slip region at thetime of the shifting-down operation to be narrower than that at the timeof shifting-up operation, at least one of a time difference between theoperating-hydraulic-pressure-increase start time for theengagement-objective clutch and theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch and a time-transitional decrease propertyin the operating pressure for the disengagement-objective clutch iscontrolled to vary depending on whether the speed-changing operation isa shifting-up operation or a shifting-down operation. For example, thetime difference is set to be relatively short.

[0022] In this case, it is preferable that, during speed-changing,regardless of variations in the time difference and thetime-transitional decrease property that have been set to meet theaforementioned individual conditions, theoperating-hydraulic-pressure-decrease start time for thedisengagement-objective clutch be set to be later than theoperating-hydraulic-pressure-increase start time at which the fluidchamber of the engagement-objective clutch becomes full of fluid, andthe pressure thereof rises to the piston-holding pressure.

[0023] Another object of the present invention is to provide anappropriate method to detect an abnormal clutch to prevent theoccurrence of a double-transmission state in the hydraulic power shiftspeed change mechanism for which the hydraulic-pressure control isperformed as described above.

[0024] To achieve this object, a pressure-detecting means is provided todetect an operating hydraulic pressure for each of the hydraulicclutches, and when the number of the pressure-detecting means fordetecting hydraulic pressures higher than a specific pressure value isgreater than the number of the hydraulic clutches to be engaged at thetime of speed-changing operation (in the speed change unit configured ofthe plurality of hydraulic type speed change units that are connected toin tandem, when two or more units of the detecting means each detect apressure higher than a specific pressure value in at least in one of thehydraulic type speed change units), one of two hydraulic-pressurecontrol operations is performed, one hydraulic-pressure controloperation being performed to engage only those of the hydraulic clutcheswhich have immediate-previously been disengaged, and the other onehydraulic-pressure control operation being performed to disengage allthe hydraulic clutches.

[0025] The individual pressure-detecting means may be configured suchthat the individual means constitute switches each turning ON or OFFwith respect to the border of the specific pressure value, and when thenumber of the pressure-detecting means for detecting hydraulic pressureshigher than a specific pressure value is greater than the number of thehydraulic clutches to be engaged at the time of speed-changing operation(in the speed change unit configured of the plurality of hydraulic typespeed change units that are connected to in tandem, when two or moreunits of the detecting means each detect a pressure higher than aspecific pressure value in at least in one of the hydraulic type speedchange units), one of two hydraulic-pressure control operations isperformed, one hydraulic-pressure control operation being performed toengage only those of the hydraulic clutches which haveimmediate-previously been disengaged, and the other onehydraulic-pressure control operation being performed to disengage allthe hydraulic clutches.

[0026] Still another object of the present invention is to detectwhether a load is imposed on a vehicle by using appropriate detectingmeans, not by relying on operator-performing switch operations. Thisallows operating hydraulic pressures for the individual hydraulicclutches to be appropriately increased without failure.

[0027] To achieve this object, in the present invention tractional-loaddetecting means is provided in a vehicle employing the speed changemechanism to thereby modify a time-transitional increase property in theoperating pressure for the hydraulic clutch to be engaged at the time ofspeed-changing and a time-transitional decrease property in theoperating pressure for the hydraulic clutch to be disengaged at the timeof speed-changing depending on whether or not the tractional-loaddetecting means detects a tractional load. Alternatively, when agovernor mechanism capable of performing control of an engine revolutionfrequency according to detection of an engine load is provided in thevehicle employing the speed change mechanism, the governor is used tomodify a time-transitional increase property in the operating pressurefor the hydraulic clutch to be engaged at the time of speed-changingdepending on whether or not the governor mechanism detects an engineload equal to or higher than a specific level.

[0028] The above load detection may be used to modify atime-transitional decrease property in the operating pressure for thehydraulic clutch to be disengaged at the time of speed-changing.

[0029] As summarized above, the speed change mechanism comprisinghydraulic clutch according to the present invention, corresponding tothe various conditions modifies the time difference between theoperating-hydraulic-pressure-increase start time for theengagement-objective clutch, the operating-hydraulic-pressure-decreasestart time for the disengagement-objective clutch, and thetime-transitional decrease property in the operating pressure for thedisengagement-objective clutch at the time of speed-changing operation.Therefore, in order to allow input/output pressures of operating fluidfed to each of the hydraulic clutches to be adjustable, the individualhydraulic clutch is controlled by means of an electromagnetic pressureproportion valve provided for each of the hydraulic clutches.

[0030] The above and other objects, configurations, and advantages ofthe invention will become apparent from the following detaileddescription thereof taken in conjugation with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a skeletal view of a tractor transmission system havinga nine-step-variable-speed-type hydraulic primary speed change mechanism1;

[0032]FIG. 2 is a diagram of a hydraulic-clutch-controlling hydrauliccircuit in the primary speed change mechanism 1;

[0033]FIG. 3 is a block diagram of an embodiment of an electricalcontroller circuit in the primary speed change mechanism 1;

[0034]FIG. 4 is a block diagram of another embodiment of an electricalcontroller circuit;

[0035]FIG. 5 is a skeletal view of a tractor transmission system havinga six-step-variable-speed-type hydraulic primary speed change mechanism1′;

[0036]FIG. 6 is a diagram of a hydraulic-clutch-controlling hydrauliccircuit in the primary speed change mechanism 1′;

[0037]FIG. 7 is a side view of a tractor employing the electricalcontroller system shown in FIG. 4;

[0038]FIG. 8 is a plan view of the aforementioned tractor;

[0039]FIG. 9 is a plan view of a primary-speed change hydraulic valveunit 3 for the primary speed change mechanism 1′;

[0040]FIG. 10 is a time-transitional hydraulic pressure graph showing apressure-increase property for an engagement-objective clutch;

[0041]FIG. 11 is a time-transitional hydraulic pressure graph showing apressure-decrease property for a disengagement-objective clutch;

[0042]FIG. 12 is a time-transitional voltage graph regarding left andright draft sensors 112R and 112L functioning as load-detecting meansfor pressure-increase-property determination;

[0043]FIG. 13 is a flowchart of pressure-increase-property determinationusing a draft sensor 122 and a traction sensor 123 that are disposed onthe left and right sides;

[0044]FIG. 14 is a time-transitional graph of engine revolutionfrequencies detected for the pressure-increase-property determination;

[0045]FIG. 15 is a time-transitional graph of rack positions detectedfor the pressure-increase-property determination;

[0046]FIGS. 16A and 16B show a flowchart of thepressure-increase-property determination using an electronic governor;

[0047]FIG. 17 shows time-transitional graphs regarding input voltagesfrom hydraulic-clutch-operating hydraulic pressures and individualpressure sensors in a first hydraulic type speed change unit 17′ and asecond hydraulic type speed change unit 20 in the primary speed changemechanism 1′, the graph concurrently showing speed changes between afirst speed position and a second speed position;

[0048]FIG. 18 shows time-transitional graphs, which are similar to theabove, regarding speed changes between the second speed position and athird speed position;

[0049]FIG. 19 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in arated-speed engine revolution state;

[0050]FIG. 20 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shift-down time in therated-speed engine revolution state;

[0051]FIG. 21 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in alow-speed engine revolution state;

[0052]FIG. 22 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shift-down time in a low-speedengine revolution state;

[0053]FIG. 23 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where a delay time is notchanged depending whether one or two disengagement/engagement clutchesare operated;

[0054]FIG. 24 shows time-transitional graphs, which are similar to theabove, regarding hydraulic-clutch-operating pressures at a shift-downtime;

[0055]FIG. 25 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where ahydraulic-pressure-increase start time of an engagement-objective clutchis controlled to match a hydraulic-reduction start time of adisengagement-objective clutch;

[0056]FIG. 26 shows time-transitional graphs regardinghydraulic-clutch-operating pressures at a shifting-up time in therated-speed engine revolution state in a case where a delay time setbetween the pressure-reduction start time of the disengagement-objectiveclutch and the pressure objective start of the engagement-objectiveclutch is controlled shorter than the delay time set in the case shownin FIG. 19;

[0057]FIG. 27 is a flowchart for detection of an abnormal clutch and forhydraulic-pressure control of the hydraulic clutch according thereto;and

[0058]FIG. 28 shows diagram of other hydraulic circuits in the primaryspeed change mechanism 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0059]FIG. 1 shows a transmission system used for a work vehicle(tractor) that has a nine-step-variable-speed-type speed changemechanism (a so-called hydraulic power shift speed change mechanism)including hydraulic clutches as a primary speed change mechanism. Thetransmission system is configured such that a travel transmission systemand a PTO transmission system are separated from an engine shaft 12connected through a buffering coupler 11 to an engine 10 disposed on aforemost portion of the vehicle as shown in FIG. 4. The transmissionsystem is housed in a transmission housing 2 shown in FIG. 4.

[0060] Hereinbelow, the transmission system shown in FIG. 1 will bedescribed; and first, the travel transmission system will be described.A hydraulic reverser unit 14 is disposed between a reverser output shaft13, which is disposed parallel to the engine shaft 12, and the engineshaft 12. A first drive shaft 15 is disposed along an extending line ofthe reverser output shaft 13, and is integrally connected to thereverser output shaft 13. A tube-type first speed change shaft 16 isdisposed on an extending line of the engine shaft 12. A first hydraulictype speed change unit 17 is provided between the first drive shaft 15and a first speed change shaft 16. A tube-type second drive shaft 18 isdisposed along an extending line of the first speed change shaft 16. Asecond speed change shaft 19 is disposed along an extending line of thefirst drive shaft 15. A second hydraulic type speed change unit 20 isprovided between the second drive shaft 18 and the second speed changeshaft 19. A propeller shaft 22 is disposed on extending line of thesecond speed change shaft 19. A mechanical speed change mechanism 23 isdisposed as a secondary speed change mechanism between the second speedchange shaft 19 and the propeller shaft 22. A small bevel gear 24engages a large input bevel gear 26 of a left/right rear-wheeldifferential mechanism 25. A differential output shaft 27 on theleft/right of the differential mechanism 25 is connected to a left/rightrear wheel 30 shown in FIG. 4 through a left/right brake 28 and finalspeed-reduction device 29 of a planetary gear type. A diff-lock clutch31 is provided on one of the differential output shafts 27.

[0061] In the travel transmission system, the primary speed changemechanism 1 is constituted by combining the first hydraulic type speedchange unit 17 and the second hydraulic type speed change unit 20.However, before it is described, the hydraulic reverser unit 14 and thespeed change mechanism 23 will hereinbelow be described in detail.

[0062] In the hydraulic reverser unit 14, a forward gear train 91 and abackward gear train 92 including an idle gear 92 a are provided betweenthe engine shaft 12 and the reverser output shaft 13. In the individualgear trains 91 and 92, gears are disposed to be idle on the engine shaft12. One of these gears on the engine shaft 12 is connected to the engineshaft 12 by alternative connection through one of a forward hydraulicclutch 14F and a backward hydraulic clutch 14R. Thereby, the forward orbackward rotation is selectively transmitted to the reverser outputshaft 13.

[0063] The speed change mechanism 23 allows a countershaft 21 to beconnected to the second speed change shaft 19 via a reduction geartrain. Two speed change gears 93 and 94 are immobilized on thecountershaft 21. Via a reduction gear mechanism 95, the speed changegear 94 on a smaller diameter side of the shaft is connected to a speedchange gear 96 disposed outside of the countershaft 21. On the otherhand, on the propeller shaft 22, gears 97, 98, and 99 are provided to beidle, and in addition, two dual-type clutches 100 function toselectively connect one of the gears 98 and 99 to the propeller shaft22. The dual-type clutch 101 provides one of two selectable connections,one of the connections connects the gear 97 to the propeller shaft 22,and the other connection directly connects the second speed change shaft19 and the propeller shaft 22 together. As described above, four-stepspeed change can be implemented according to the mechanical speed changemechanism that functions as the secondary speed change mechanism.

[0064] The aforementioned tractor can travel either by two-wheel drivingof the left and right rear wheels 30 to which power is transmittedthrough the travel transmission system or by four-wheel driving in whichleft and right front wheels 6 shown in FIG. 4 are also selectivelydriven. In a transmission system for the front-wheel driving power, twogears 34 and 35 are idly provided on a countershaft 33 to be integrallyand relatively rotatable thereon. A gear 32 immobilized on the propellershaft 22 engages the gear 34; and gears 36 and 37 engage the gears 34and 35, respectively. Between the gears 36 and 37 and a driving-powertaking off shaft 38, a hydraulic clutch unit 39 is provided toselectively connect one of the gears 36 and 37 to thedriving-power-taking-off shaft 38. When the gear 37 is connected to thedriving-power-taking-off shaft 38, the front wheels 6 and the rear wheel30 rotate at a synchronizing speed. Also, when the gear 36 is connectedthereto, the front wheels 6 rotate at a speed higher that of the rearwheels 30.

[0065] Hereinbelow, the PTO transmission system will be described. Atransmission shaft 40 extends from the rear end of the engine shaft 12,passing through the tubular-type first speed change shaft 16, seconddrive shaft 18, and countershaft 21. A transmission shaft 41 extendsfrom the rear end of the transmission shaft 40. A PTO clutch 42 isprovided between the transmission shaft 41 and a transmission shaft 43that is provided on an extending line of the transmission shaft 41. APTO shaft 44 is disposed parallel to the transmission shaft 43 to extendoutside of the mechanism. Inside the mechanism, a mechanical PTO speedchange device 45 is provided between the transmission shaft 43 and thePTO shaft 44. Through gears 46, 47, and 48, the transmission shaft 41transmits power to a power-taking-off shaft 49 to drive a hydraulic pump50. The hydraulic pump 50 discharges pressurized fluid is used tooperate hydraulic clutches of the first hydraulic type speed change unit17 and the second hydraulic type speed change unit 20. In this case,fluid discharging from the hydraulic pump 50 may be used to verticallymove a hydraulic work-machine lifting device provided in a rear portionof the tractor.

[0066] Hereinbelow, the primary speed change mechanism 1 in the traveltransmission system will be described in detail. In the first hydraulictype speed change unit 17, three gears 51, 52, and 53 are provided to beidle on the first drive shaft 15, and respectively engage three gears54, 55, and 56 immobilized on the first speed change shaft 16. Therespective gears 51, 52, and 53 are alternatively connected to the firstdrive shaft 15 through three hydraulic clutches 57, 58, and 59 providedon the first drive shaft 15 to thereby allow three-step speed changes tobe implemented.

[0067] In the first hydraulic type speed change unit 20, three gears 60,61, and 62 are provided to be idle on the first drive shaft 18, andrespectively engage three gears 63, 64, and 65 immobilized on the firstspeed change shaft 19. The respective gears 63, 54, and 65 arealternatively connected to the first drive shaft 19 through threehydraulic clutches 66, 67, and, 68 provided on the first drive shaft 19to thereby allow three-step speed changes to be implemented.

[0068] The primary speed change mechanism 1 is configured to include thefirst hydraulic type speed change unit 17 and the second hydraulic typespeed change unit 20 that are connected together in tandem. When one ofthe hydraulic clutches 57, 58, and 59 is connected to one of thehydraulic clutches 66, 67, and 68, totally, nine-step speed changes canbe obtained.

[0069] As shown in Table 1, according to combinations of alternativeconnections of the hydraulic clutches 57, 58, and 59 and alternativeconnections of the hydraulic clutches 66, 67, and 68, the first andsecond hydraulic type speed change units 17 and 20 are set so as toobtain first to ninth speed ratios (output rotation speed/input rotationspeed; i.e., the rotation speed of the second speed change shaft19/rotation speed of the first drive shaft 15) TABLE 1 Hydraulicclutches Hydraulic clutches connected in the connected in the firsthydraulic type second hydraluic type Speed ratios speed change unit 17speed change unit 20 1st 57 66 2nd 58 66 3rd 59 66 4th 57 67 5th 58 676th 59 67 7th 57 68 8th 58 68 9th 59 68

[0070] Hereinbelow, a description will be made regarding a primaryspeed-changing hydraulic circuit shown in FIG. 2. The circuit isprovided for operating the hydraulic clutches 57, 58, and 59 in thefirst hydraulic type speed change unit 17, and the hydraulic clutches66, 67, and 68 in the second hydraulic type speed change unit 20. Thehydraulic pump 50 shown in FIG. 1 discharges fluid having a hydraulicpressure set by a pressure-controller valve 69 to a fluid-feeder circuit70.

[0071] The fluid-feeder circuit 70 is separated to branch circuits thatare connected to the aforementioned six hydraulic clutches 57, 58, 59,66, 67, and 68. In the individual branch circuits, two-position-methodelectromagnetic proportion selector valves VL, VM, VH, V1, V2, and V3are provided. For the convenience of description, a variable aperture Vaformed in each of the electromagnetic proportion selector valves isshown outside of each of the electromagnetic proportion selector valves.

[0072] Solenoids SL, SM, SH, S1, S2, and S3 of the respectiveelectromagnetic proportion selector valves VL, VM, VH, V1, V2, and V3are each controlled to an operational position through excitation. Theyare each controlled to a neutral position when nonexcited. That is, wheneach of the solenoids is excited, a hydraulic clutch correspondingthereto is controlled to engage; whereas, when it is relieved fromexcitation, a hydraulic clutch corresponding thereto is controlled todisengage.

[0073] Pressure sensors PSL, PSM, PSH, PS1, PS2, and PS3 are connected,respectively, between the electromagnetic proportion selector valves VL,VM, VH, V1, V2, and V3 in the respective branch circuits and thehydraulic clutches 57, 58, 59, 66, 67, and 68. These pressure sensorsdetecting operating hydraulic pressures may be each connected to aswitch that performs ON/OFF operations relative to a predeterminedpressure value. Alternatively, the pressure sensor itself may beconstructed as the above sensor. Pressure sensors shown in FIGS. 17, 18,and 27 are individually configured as switches that turn ON when apressure value equal to or greater than a predetermined pressure value(switching pressure pb described below) and that turn OFF when thepressure value is lower than the value. However, they may be configuredto turn OFF when the pressure value is lower than the predeterminedpressure value and to turn ON when the pressure value is higher than thevalue.

[0074] The above-described electromagnetic proportion selector valvesand pressure sensors are all stored in the primary-speed-changehydraulic valve unit 3, as shown in FIG. 9. They are disposed in a partof the tractor, as shown in FIG. 8, and are connected tohydraulic-clutches in the transmission housing 2 through pipings. FIG. 9shows the primary-speed-change hydraulic valve unit 3 that storeselectromagnetic proportion selector valves and pressure sensors of theprimary speed change mechanism 1′, which will be described below. Whilethe primary speed change mechanism 1′ will be described below in detail,it is briefed hereinbelow. It is configured by tandem connection of thefirst hydraulic type speed change unit 17′ and the second hydraulic typespeed change unit 20. The first hydraulic type speed change unit 17′ isconfigured by eliminating an intermediate-speed clutch 58 from the firsthydraulic type speed change unit 17 in the nine-step primary speedchange mechanism 1. The second hydraulic type speed change unit 20 isthe same as that in the primary speed change mechanism 1. Because ofthis configuration, the train of the above-described electromagneticproportion selector valves and the train of the above-described pressuresensors are stored in the primary-speed-change hydraulic valve unit 3 ina state where the electromagnetic proportion selector valve VM (and thesolenoid SM) and the pressure sensor PSM are removed.

[0075] A description will hereinbelow be made referring to back to thehydraulic circuit diagram in FIG. 2. A lubricant-pressure-settingsecondary pressure controller valve 72 is connected to a drain side of apressure-controller valve 69 that branches from the fluid-feeder circuit70. A lubricant circuit 73 is led from a portion between the twopressure controller valves 60 and 72 to feed lubricant to the hydraulicclutches 57, 58, 59, 66, 67, and 68.

[0076] A line filter 76 and a relief valve 77 that functions as a bypassvalve are parallel-connected to a fluid-drawing-in circuit 75 thatextends from a fluid reservoir 74 up to the hydraulic pump 50. When theline filter 76 is incidentally blinded, the relief valve 77 performs arelief operation to maintain lubricant to flow to the hydraulic pump 50.

[0077] A hydraulic pump 78 driven by the engine shaft 12, as shown inFIG. 1, discharges fluid to the two hydraulic clutches 14F and 14R ofthe above-described hydraulic reverser unit 14. A fluid-drawing-incircuit 79 is provided to connect a fluid-drawing-in side of thehydraulic pump 78 and the fluid-drawing-in circuit 75 to also feed fluidin the fluid reservoir 74 to the hydraulic clutches 14F and 14R.

[0078] Hereinbelow, referring to FIG. 3, a description will be maderegarding electrical operation control of the electromagnetic proportionselector valves VL, VM, VH, V1, V2, and V3 for the primary speed changemechanism 1. An input-side interface of a logical circuit 80, as shownin FIG. 8, in a controller 4 disposed in part of the tractor isconnected to a potentiometer 82, a tachometer 83, a mode-selector switch84, and the above-described six pressure sensors PSL, PSM, PSH, PS1,PS2, and PS3. The potentiometer 82 detects the position (lever angle)and the rotational direction of a primary speed change lever 81 disposednear an operator seat 7, as shown in FIGS. 7 and 8. The speeds are setin the range of first to ninth speed positions in the order from lowerto higher speeds; and as shown in FIG. 3, numbers 1 to 9 correspondingto the speed positions are indicated in a rotational area of the primaryspeed change lever 81. The tachometer 83 detects the revolutionfrequency of the engine 10. The mode-selector switch 84 functions inresponse to an operation performed by an operator to vary ahydraulic-pressure-increase property that causes the hydraulic clutchesin the primary speed change device to engage corresponding to a normal(on-the-road) travel mode and a work traveling mode that is to becarried out at a tractional load. An electronic governor may also beused as a selector switch between two control modes. One of the controlmodes is carried out to adjust the frequency of the engine revolution toa revolution frequency at the time of normal acceleration operation. Theother control mode is carried out to detect engine load ratios, andcontrols the engine revolution frequency corresponding to the loadratios.

[0079] However, incidents can occur in which an operator overlooksoperations of the mode-selector switch 84, the traction load actuallyexerted is not greater than a logical load even when the above-describedswitch is set to the work traveling mode, thus causing ahydraulic-pressure-increase property that is different from an actualproperty. Taking the above into account, two embodiments of thehydraulic pressure control are disclosed below with reference to FIGS.12 and 13 and FIGS. 14 to 16. The individual embodiment include anautomatic load detection structure that is capable of makingself-determination as to which one of the control modes should beselected to allow the hydraulic-pressure-increase property for thehydraulic clutches to be selected corresponding to actual states. Inconjunction with these control modes, there are provided left and rightdraft sensors 122L and 122R L to right and left lower links 121 in awork-machine-attaching device 120, a traction sensor 123 on a draw-bar(not shown), and an electronic governor controller 5. FIG. 4 disclosesan electrical controller circuit. In the circuit, instead of themode-selector switch 84, the left and right draft sensors 122L and 122R,the traction sensor 123, and the electronic governor controller 5, whichare input means for hydraulic-clutch pressure-increase-propertyselection, are connected to the input-side interface of the logicalcircuit 80. A tachometer 83 and a rack-position sensor 124 are connectedto the input side of the logical circuit 80. Through the electronicgovernor controller 5, the logical circuit 80 receives a signal inputfrom the tachometer 83, and in addition, a load-ratio signal obtainedthrough calculation performed according to signals input from thetachometer 83 and the rack-position sensor 124. In addition, theelectronic governor controller 5 is connected to a hydraulic liftcontroller 125 for hydraulically driving the work-machine-attachingdevice 120 and to an electronic governor 126 (a driving device for afuel-injection-amount controller rack).

[0080] In each of the electrical controller circuits shown in FIGS. 3and 4, an output side of the logical circuit 80 is connected to a delaycircuit 88 that is connected to the input side of a solenoid-drivercircuit 86 and to a solenoid-driver circuit 85. The solenoid-drivercircuit 85 drives solenoids SL, SM, SH, S1, S2, and S3 of theelectromagnetic proportion selector valves VL, VM, VH, V1, V2, and V3 inan exiting direction. The solenoid-driver circuit 86 drives thesesolenoids to be relived from excitation.

[0081] The output side of the logical circuit 80 is connected to thesolenoid-driver circuit 85 and a pressure (-increase-property)-settingcircuit 87. The output side of the pressure-setting circuit 87 isconnected to the solenoid-driver circuit 85. The pressure-settingcircuit 87 stores two types of solenoid excitation patterns that areused to obtain two types of pressure-increase properties as representedby pressure-increase graphs U1 and U2 shown in FIG. 10.

[0082] In addition, the output side of the logical circuit 80 isconnected to the delay circuit 88, a time-setting circuit 90, and apressure (-decrease-property)-setting circuit 89. The output side of thetime-setting circuit 90 is connected to the delay circuit 88. Thesolenoid-driver circuit 86 is connected to the output side of the delaycircuit 88 and to the output side of the pressure-setting circuit 89,and to the solenoid-driver circuit 86. The pressure-setting circuit 89stores three types of solenoid-excitation-relieving patterns that areused to obtain three types of pressure-decrease properties asrepresented by pressure-decrease graphs D1, D2, and D3 shown in FIG. 11.

[0083] In the logical circuit 80, an engagement-objective clutch anddisengagement-objective clutch are determined according to a signal thatrepresents postshift position of the primary speed change lever 81,which is detected through the potentiometer 82. In addition, accordingto a logic described below, a hydraulic-pressure-decrease property forthe disengagement-objective clutches are determined. The electricalcontroller circuit shown in FIG. 3 performs setting through themode-selector switch 84. On the other hand, the electrical controllercircuit shown in FIG. 4 inputs signals from the right and left draftsensors 122 and the traction sensor 123. The circuit determines apressure-increase property for engagement-objective clutches accordingto inputs from the electronic governor controller 5. In addition,according to inputs from the pressure sensors PSL, PSM, PSH, PS1, PS2,the circuit determines the necessity for control that is performed toprevent entrance of foreign substances to the hydraulic clutches.

[0084] The logical circuit 80 sends a signal to the solenoid-drivercircuit 85. This signal causes the solenoid-driver circuit 85 to send anON-signal to a solenoid for an objective electromagnetic proportionselector valve. Concurrently, the logical circuit 80 sends to thepressure-setting circuit 87 a pressure-setting signal for selecting oneof the solenoid excitation patterns. Thereby, control is performed fortransmission of the ON-signal to the solenoid according to the solenoidexcitation pattern that has been set in the pressure-setting circuit 87.

[0085] Similarly, the logical circuit 80 sends a signal to thesolenoid-driver circuit 86, and the signal causes the solenoid-drivercircuit 86 to send an OFF-signal to a solenoid for an objectiveelectromagnetic proportion selector valve. Concurrently, the logicalcircuit 80 sends to the pressure-setting circuit 89 a pressure-settingsignal for selecting one of the solenoid nonexcitation patterns.Thereby, control is performed for transmission of the OFF-signal to thesolenoid according to the solenoid excitation pattern that has been setin the pressure-setting circuit 89.

[0086] In addition to the logical circuit 80, similar electricalcontroller circuits 85, 86, 87, 88, 89, and 90 are provided either inthe above-described controller 4 or in the primary-speed-changehydraulic valve unit 3. According to the solenoid-driver circuit 85 andthe solenoid-driver circuit 86, control signals (ON/OFF signals) aresent to objectives of the solenoids SL, SM, SH, S1, S2, and S3 of theelectromagnetic proportion selector valves VL, VM, VH, V1, V2, which areprovided in the primary-speed-change hydraulic valve unit 3.

[0087] Hereinbelow, a description will be made regarding thetransmission system for the work vehicle (tractor) equipped with theprimary speed change mechanism 1′ of the six-step-speed-change type,which is shown in FIG. 5.

[0088] Individual components and constructions in the transmissionsystem are the same as those shown in FIG. 1, except for the firsthydraulic type speed change unit 17′. The first hydraulic type speedchange unit 17′ shown in FIG. 1 is configured to exclude theintermediate-speed-step gear train, that is, the gears 52 and 55, tothereby enable two-step speed changes. In addition, with the overallprimary speed change mechanism 1′ configured to include the combinationof the first hydraulic type speed change unit 17′ and the secondhydraulic type speed change unit 20, totally six-step speed changes areenabled.

[0089] Specifically, as shown in FIG. 2, according to combinations ofalternative connections of the hydraulic clutches 57 and 59 andalternative connections of the hydraulic clutches 66, 67, and 68, thefirst and second hydraulic type speed change units 17′ and 20 are set soas to obtain first to sixth speed ratios (output rotation speed/inputrotation speed; i.e., the rotation speed of the second speed changeshaft 19/rotation speed of the first drive shaft 15) TABLE 2 Hydraulicclutches Hydraulic clutches connected in the connected in the firsthydraulic type second hydraulic type Speed ratios speed change unit 17speed change unit 20 1st 57 66 2nd 59 66 3rd 57 67 4th 59 67 5th 57 686th 59 68

[0090]FIG. 6 shows a hydraulic-clutch-controlling hydraulic circuit inthe primary speed-changing mechanism 1′ shown in FIG. 5. The samereference numerals/symbols as those shown in FIG. 2 represent the samemembers shown therein. Although an electrical controller circuit is notdisclosed therein, it is configured such that the pressure sensor PS andthe solenoid SM are removed from the electrical controller circuit shownin FIG. 4 or 5, and first to sixth speed positions of a primary speedchange lever 81 therein are included.

[0091]FIGS. 7 and 8 each show the tractor employing the electricalcontroller system shown in either FIG. 1 or FIG. 5. The members shownwith the reference numerals have already been described in connectionwith the transmission system shown in FIGS. 1 to 4. The configurationincludes a load-detecting means used in determination of apressure-increase property for engagement-objective clutches, andemploys the electrical controller circuit shown in FIG. 4 rather thanthat shown in FIG. 3. In addition, the primary-speed-change hydraulicvalve unit 3 shown in FIG. 9 is disposed in the position shown in FIG.8; and as described above, it is intended for the six-step-type primaryspeed change mechanism 1′ shown in FIG. 5. To use it for thenine-step-type primary speed change-mechanism 1 shown in FIG. 1, theconfiguration may be modified such that the valve device is replacedwith a valve device in which the electromagnetic proportion selectorvalve VM and the pressure sensor PSM are added and stored.

[0092] Hereinbelow, a description will be made regarding the hydraulicpressure control in the hydraulic-clutch-included speed change mechanismof the present invention. The hydraulic pressure control described belowmay be applied either to the nine-step-type primary speed changemechanism 1 shown in FIG. 1 or to the six-step-type primary speed changemechanism 1′ shown in FIG. 5.

[0093]FIG. 10 shows a pressure-increase property for anengagement-objective clutch at the time of speed-changing. Specifically,from an engagement start time t₀ when an ON-signal is fed to anobjective solenoid (excitation is started), a clutch-operating hydraulicpressure p is gradually increased to finally reach a normal hydraulicpressure p₁, as shown by pressure-increase graphs U1 and U2.

[0094] In the pressure-increase graphs U1 and U2, the low-levelpressure-increase graph U1 is set at a road-travel time when a travelload is low, whereas the high level pressure-increase graph U2 is set ata work-travel time when the travel load is high. When the travel load ishigh, torque transmission efficiency needs to urged to increase, and aload resistance force needs to be exerted. At the road-travel time whenthe travel load is low, since amenity is required, shock that can occuraccording to hydraulic-pressure rise at the time of clutch-shiftoperation needs to be minimized. In a configuration using the electricalcontroller circuit shown in FIG. 3, an operator uses the mode-selectorswitch 84 to determine which one of the pressure-increase graphs U1 andU2 is set. In a configuration using the electrical controller circuitshown in FIG. 4, determination regarding which one of thepressure-increase graphs U1 and U2 is set is dependent on determinationthat is made in the logical circuit 80. The determination is made in thelogical circuit 80 according to either signals input from the right/leftdraft sensors 122 and the traction sensor 123 shown in FIG. 8 or anengine-load-ratio signal that is input through the electronic governorcontroller 5.

[0095] Hereinbelow, referring to FIGS. 12 and 13, a description will bemade regarding a method for determining a pressure-increase property foran engagement-objective clutch according to load detection that isperformed using the right and left draft sensors 122L and 122R and thetraction sensor 123.

[0096] When the right/left lower link 121 is pulled backward, theright/left draft sensor 122L/122R detects a load thereof. Depending onthe pulling force, variations occur in the value of voltage input to thelogical circuit 80. In FIG. 12, the sum of output voltage values of thetwo draft sensors 122 is represented by a load voltage value L. When theload voltage value L is a value L1 at a normal (on-the-road) traveltime, the load voltage value L is a value L2, which is lower than thevalue L1, in a tractional-work travel time. A threshold value L3 is setbetween the values L1 and L2, and it is assumed that a case where theload voltage value L is equal to or lower than the threshold value L3 isrepresented as a selection zone of a pressure-increase property for aprimary-speed-change hydraulic clutch at a traction-load mode.

[0097] On the other hand, the traction sensor 123 turns OFF at a normal(on-the-road) travel time, and it turns ON upon being imposed by a loadat a tractional-work travel time. When the traction sensor 123 is turnedON, selection is made for a pressure-increase property for theprimary-speed-change hydraulic clutch at a tractional-load mode.

[0098] Specifically, as shown in a flowchart in FIG. 13, in at least oneof the cases where the load voltage value L input from the right/leftdraft sensor 122 to the logical circuit 80 is equal to or less than L3(step 201) and where the traction sensor 123 is turned ON (step 202),the tractional-load-mode pressure-increase graph U2 is selected (step204). In the other case, i.e., when the load voltage value L of theright/left draft sensor 122L/122R is turned OFF, and concurrently, thetraction sensor 123 is turned OFF, the normal-travel-modepressure-increase graph U1 is selected (step 203).

[0099] Hereinbelow, referring to FIGS. 14 to 16, an introduction is maderegarding another embodiment that determines a pressure-increase patternaccording to detection by the electronic governor of the engine forengine loads (load ratios). The embodiment of the control may beemployed by a vehicle mounting a diesel engine corresponding to theengine 10.

[0100] As shown in FIG. 4, the electronic governor controller 5 isconnected to the input-side interface of the logical circuit 80 tothereby allow load-ratio signals in the electronic governor controllerto be input to the logical circuit 80. The electronic governorcontroller 5 is connected to a hydraulic-pressure lift controller 125,the above-described tachometer 83 that detects the engine revolutionfrequency, and the rack-position sensor 124 that detects the position ofthe fuel-injection-amount controller rack of the governor.

[0101] The electronic governor controller 5 calculates load ratiosaccording to inputs from the tachometer 83 and the rack-position sensor124. In addition, it inputs an engine-load-ratio signal, which is outputbased-on the load ratios, to the hydraulic-pressure lift controller 125to thereby use engine-load-ratio signal to lift a hydraulic lift of thework-machine-attaching device 120. Concurrently, the electronic governorcontroller 5 inputs an engine-load-ratio signal, uses an output controlsignal fed back from the logical circuit 80 to move the rack, andthereby controls the fuel-injection amount. Among theseengine-load-ratio signals issued from the electronic governor controller5, the signal for the hydraulic-pressure lift controller 125 is outputat a long frequency to prevent overcontrol that can reduce workefficiency. The signal for the logical circuit 80 is output at a shortfrequency so that the engine revolution frequency can be quicklycontrolled corresponding to the load. By use of the signal output to thelogical circuit 80 at the short frequency without performingmodification, pressure-increase properties can be quickly determinedcorresponding to load ratios, thereby allowing the signal to beeffectively used for hydraulic-pressure control of the hydraulic clutch.

[0102] As shown in FIG. 14, an engine revolution frequency Nerepresented by a voltage input from the tachometer 83 to the electronicgovernor controller 5 is assumed to be changed from Ne3 to Ne1 that islower than Ne3. On the other hand, as shown in FIG. 15, a rack positionLs, of which data has been input from the rack-position sensor 124 tothe electronic governor controller 5, is assumed to be changed from Ls3to Ls1 (on a side where the fuel-injection amount is relatively large)that is higher than Ls3. In this way, when the reduction in the enginerevolution frequency and the lifting-up (i.e., increase in thefuel-injection amount) concurrently occur, and in addition, theindividual reduction and lifting-up appear with specific properties,determination is made such that a tractional load is imposed on thevehicle, and the pressure-increase graph U2 is set in thepressure-setting circuit 87.

[0103] Hereinbelow, referring to FIGS. 16A and 16B, a description willbe made regarding a pressure-increase-property setting flow that iscarried out based on detection of the engine revolution frequency andthe rack position. First, as prerequisite processing, the tachometer 83detects engine revolution frequencies Ne at a specific short frequency,the rack-position sensor 124 detects rack positions Ls at the samefrequency as that for the engine revolution frequencies Ne, and valuesof the detections are serially stored therein. In step 301, among enginerevolution frequencies Ne serially detected, the circuit stores at leastengine revolution frequencies Ne2 (immediate-previously detectedrevolution frequency), Ne3 (second-previously detected revolutionfrequency), Ne4 (third-previously detected revolution frequency), andNe5 (fourth-previously detected revolution frequency). Concurrently (instep 312 in the flow for the convenience of description), among rackpositions Ls serially detected, the system stores at least rackpositions Ls2 (immediate-previously detected position at t₂), Ls3(second-previously detected position at t₃), Ls4 (third-previouslydetected position at t₄), and Ls5 (fourth-previously detected positionat t₅).

[0104] Then, a new engine revolution frequency Ne is detected by thetachometer 83 at a current detection start time t₁, and a signalrepresenting a currently detected engine revolution frequency Ne1 isinput to the electronic governor controller 5 (step 302). Then, theengine revolution frequency Ne2 immediate-previously detected isretrieved, and the current engine revolution frequency Ne1 is comparedto the previous Ne2 to verify whether the reduction in the intendedengine revolution frequency has been achieved; that is, it verifieswhether Ne1<Ne2 has been achieved (step 303). After the verification ofthe reduction in the engine revolution frequency, a calculation of areduction amount al (=Ne2−Ne1) is performed (step 304). In addition,data of the stored engine revolution frequencies Ne2, Ne3, Ne4, and Ne5is retrieved, and verification is performed for at least the reductionin the engine revolution frequency from the fourth-previously detectedfrequency. Subsequently, calculations are performed to obtain reductionamounts a2 (=Ne3−Ne2), a3 (=Ne4−Ne3), and a4 (=Ne5−Ne4) (steps 305 to310) to thereby verify whether a1−a2≧a3−a4, that is, the increase in thereduction ratio of the engine revolution frequency, has been achieved(step 11).

[0105] If the engine revolution frequency is reduced, and the reductionamount per unit time in that case is increased, it is conceivable thatthe speed has been reduced because of either acceleration setting or atractional load. If the speed has been reduced because of theacceleration setting, the rack position in an electronic governor 126 issupposed to be at a fuel-injection-amount reduced side (a rack-positiondetection voltage should have been reduced). On the other hand, in acase where the engine revolution frequency has been reduced, but therack position has been shifted to a fuel-injection-amount increased side(the rack-position detection voltage is increased) despite of the factthat the engine revolution frequency has been reduced, the case can bedetermined that the electronic governor 126 has performed controlcorresponding to the load.

[0106] Under the above concepts, the Ls2, Ls3, Ls4, and Ls5 are stored(step 312), as described above. In-this state, a new rack position Ls isdetected by the rack-position sensor 124 at a current detection starttime t₁, and a signal representing a detected rack position Ls1 is inputto the electronic governor controller 5 (step 313). Then, data of theengine revolution frequency Ls2 immediate-previously detected isretrieved, and the current rack position Ls1 is compared to the previousLs2 to thereby verify whether the rack position has been lifted up(shifted to a fuel-injection-amount increased side), that is, to verifywhether Ls1>Ls2 has been achieved (step 314). After the verification ofthe lift-up in the rack position, a calculation of a reduction amount b1(=Ls1−Ls2) is performed (step 315). In addition, stored data of the rackpositions Ls2, Ls3, Ls4, and Ls5 is retrieved, and verification isperformed for at least the reduction in the engine revolution frequencyfrom the fourth-previously detected result. Subsequently, calculationsare performed to obtain reduction amounts b2 (=Ls3−Ls2), b3 (=Ls4−Ls3),and b4 (=Ls5−Ls4) (steps 316 to 321) to thereby verify whetherb1−b2≧b3−b4, that is, the increase in the increase-shift ratio regardingthe rack position, is achieved (step 322).

[0107] In this way, in a case where the reduction in the enginerevolution frequency and the lift-up in the rack position concurrentlyoccur, and the individual variations are abrupt, the case is determinedto be a loaded state, and a solenoid-exciting pattern for implementingcontrol represented by the tractional-work-intended pressure-increasegraph U2 is set in the pressure-setting circuit 87 (step 323). In theother case, a solenoid-exciting pattern for implementing control asrepresented by the normal(on-the-road)-travel-intended pressure-increasegraph U1 is set in the pressure-setting circuit 87 (step 324).

[0108] As described below, the pressure-decrease properties(pressure-decrease graphs) are determined such that the solenoidnonexcitation patterns in the pressure-setting circuit 89 are selectedcorresponding to engine revolution frequencies and the like detected bythe tachometer 83. In order to allow the pressure-increase property tobe modified corresponding to loads, the pressure-decrease property fordisengagement-objective clutches may also be established to be modifiedbased on engine-load-ratio signals input to the logical circuit 80 fromeither load-ratio-detecting means such as the right and left draftsensors 122 and the traction sensor 123 or the electronic governorcontroller 5.

[0109] Hereinbelow, referring back to FIG. 10, a detailed descriptionwill be made regarding the overall increase processing of an operatinghydraulic pressure p. A solenoid for an objective hydraulic clutch at aclutch-engagement start time t₀ is turned ON, and supply of fluid to afluid chamber of the objective hydraulic clutch is started. Theoperating hydraulic pressure p in the fluid chamber slightly rises atthe time t₀, and then gradually increases. At a time ta when some timehas passed from the time t₀, the fluid chamber becomes full of fluid,upon detection of arrival of hydraulic pressure p at a piston-holdingpressure p_(a) (i.e., a pressure allowing a piston to operate), thehydraulic pressure p is quickly increased to a normal pressure at thattime.

[0110] In a period up to a time tb, as shown in part of each of thepressure-increase graphs U1 and U2, the hydraulic pressure p graduallyincreases, and the clutch stays at a slip state. At the time tb, thehydraulic pressure p reaches a value required for complete engagement ofthe clutch. Subsequently, as shown in a part b of each of thepressure-increase graphs U1 and U2, the hydraulic pressure p isgradually increased up to a normal pressure p₁ to thereby cause theclutch to a pressed state. When the hydraulic pressure p reaches thenormal pressure p₁, the clutch engagement is completed.

[0111]FIG. 11 shows a hydraulic-pressure-decrease property for adisengagement-objective clutch at the time of speed-changing.Specifically, it shows cases where, from a time ts when an OFF-signal isapplied to an objective solenoid (excitation is started therefor), aclutch-operating normal pressure p₁ is reduced as represented byhydraulic-pressure-decrease property graphs D1, D2, and D3. Thehydraulic-pressure-decrease property graph D1 represents a case where,immediately after the solenoid is relieved from excitation, the pressurep is quickly reduced from the normal pressure p₁ to a piston-holdingpressure p_(a). The hydraulic-pressure-decrease property graphs D2 andD3 each represent a case where after the pressure p is quickly reducedto a pressure p_(b) that is higher than the piston-holding pressurep_(a), it is gradually increased to 0 (or the lowest value in thevicinity). Therefore, the reduction degree of D2 is greater than that ofD3.

[0112] In either case, the hydraulic clutch is urged toward a neutralposition. The overall disengagement period of the clutch, i.g., a periodin which the pressure p is changed to 0 (or the lowest value in thevicinity)(even in the case of setting of the hydraulic-pressure-decreaseproperty graph D3 representing the slowest pressure reduction) isshorter than the overall disengagement period described above.

[0113] Pressure-decrease property graphs are not limited to the threegraphs D1 to D3. The angle in the gentle-sloped pressure-decreaseportion as can be seen in either D2 or D3 may be variously set so thatcontrol as represented by other pressure-decrease property graphs can beimplemented. However, for the convenience of description, embodimentsshown in FIGS. 19 to 22, which will be described below, are assumed tohave the capacity of performing control represented by thehydraulic-pressure-decrease property graphs D1, D2, and D3.

[0114] The pressure is gradually increased, and thehydraulic-pressure-increase property is varied in the course from a to baccording to the control of an application voltage to each of thesolenoids. In addition, the variable aperture of the individualelectromagnetic proportion selector valve is used to slowly vary thehydraulic pressure as represented by the hydraulic-pressure-decreaseproperty graph D2 or D3. In the hydraulic circuit diagram shown in FIG.2, the individual variable aperture is shown in outside portion withreference symbol Va. According to the variations in the voltage appliedto the individual variable aperture Va, the amount of drain from theindividual electromagnetic proportion selector valve is controlled,thereby causing the decrease behavior of the hydraulic pressure to vary.

[0115] Hereinbelow, referring to FIGS. 17 to 26, a description will bemade regarding the relationship between the engagement course and thedisengagement course of a clutch at time of speed-changing.

[0116] First, as a basic concept, either the first hydraulic type speedchange unit 17 or the second hydraulic type speed change unit 20 isconfigured so as not to encounter total power cut during speed-changing.When one of the speed change units encounters a non-transmissible state,that is, when all hydraulic clutches in one of the speed change unitsare held in disengaged states, transmission is not performed in theprimary speed exchange device, that is, transmission is not performedbetween the first drive shaft 15 to the second speed change shaft 19. Ifwork travel was performed in the above state, the vehicle might beunexpectedly stopped, and in addition, a great shock giving discomfortwould be caused by hydraulic-pressure rise according to clutchengagement performed from the above state.

[0117] As described above, the clutch-engagement period is longer thanthe clutch-disengagement period (even with any one of thepressure-decrease patterns, being set), and theobjective-clutch-operating hydraulic pressure is gradually increased.Taking the above into account, the present invention is arranged suchthat the reduction in hydraulic pressure of a disengagement-objectiveclutch is started during gradual increase in pressure for anengagement-objective clutch. Thereby, a period in which a pressure p foroperating the disengaging clutch is controlled to be higher than thepiston-holding pressure p_(a) (a state where the clutch slips) iscontrolled to overlap a period in which a pressure p for operating theengaging clutch is higher than the piston-holding pressure p_(a) (astate where the clutch is slipping). That is, even when the transmissionefficiency is reduced to the lowest-value level, either thedisengagement-objective clutch or the engagement-objective clutch iscontrolled to slip, thereby avoiding a case where one of the clutches isforced to be in a disengaged state, and the primary speed changemechanism is forced to be in a non-transmissible state.

[0118] In this connection, for example, as shown in time-transitionalgraphs of FIGS. 17 and 19 regarding hydraulic-clutch-operating hydraulicpressures (graphs each showing a hydraulic-clutch-operating pressure prelative to a time t), regions where an in-engagement clutch and anin-disengagement clutch commonly slip (hereinbelow, the aforementionedregion will be referred to as a “common slip region”) are shown byslanting lines. The state and the area of the common slip region arepreferably set so that speed-changing (speed-position shifting) can beperformed most smoothly; that is, good speed-change feeling can besecured without being influenced by the capacity of the hydraulic pump50.

[0119] To change the speed by shifting clutches of one of the primaryspeed change mechanisms 1 and 1′ through shifting of the primary speedchange lever 81, as can be seen from Tables 1 and 2, there are twocases. In one of the cases, one hydraulic clutch is newly engaged, andanother engaged hydraulic clutch is disengaged in only one of the firsthydraulic type speed change unit 17 (17′) and the second hydraulic typespeed change unit 20. In the other case, one hydraulic clutch is newlyengaged, and another engaged hydraulic clutch is disengaged in the twofirst hydraulic type speed change units 17 (17′) and 20.

[0120] In the former case, for example, the following operations areperformed. In the primary speed change mechanism 1, when the primaryspeed change lever 81 is shifted up from the third speed position to thefifth speed position, the engaged hydraulic clutch 58 is remainedunchanged in the first hydraulic type speed change unit 17; and thehydraulic clutch 67 is newly engaged, and the engaged hydraulic clutch66 is disengaged in the second hydraulic type speed change unit 20. Whenthe primary speed change lever 81 is shifted down from the sixth speedposition to the fourth speed position, the engaged hydraulic clutch 67is remained unchanged in the second hydraulic type speed change unit 20;and the hydraulic clutch 57 is newly engaged, and the engaged hydraulicclutch 59 is disengaged in the first hydraulic type speed change unit17.

[0121] In the latter case, for example, the following operations areperformed. In the primary speed change mechanism 1, when the primaryspeed change lever 81 is shifted up from the second speed position tothe sixth speed position, the engaged hydraulic clutch 59 in the firsthydraulic type speed change unit 17 and the hydraulic clutch 67 in thesecond hydraulic type speed change unit 20 are newly engaged, and thehydraulic clutch 58 in the first hydraulic type speed change unit 17 andthe hydraulic clutch and hydraulic clutch 66 in the second hydraulictype speed change unit 20 are disengaged. When the primary speed changelever 81 is shifted down from the ninth speed position to the fifthspeed position, the engaged hydraulic clutch 58 in the first hydraulictype speed change unit 17 and the hydraulic clutch 67 in the secondhydraulic type speed change unit 20 are newly engaged, and the hydraulicclutch 59 in the first hydraulic type speed change unit 17 and thehydraulic clutch and hydraulic clutch 68 in the second hydraulic typespeed change unit 20 are disengaged.

[0122] In the primary speed change mechanism 1′, in the former case, forexample, when the primary speed change lever 81 is either shifted up orshifted down between the first speed position and the second speedposition, the hydraulic clutch 66 is remained engaged in the secondhydraulic type speed change unit 20, one of the hydraulic clutches 57and 59 is engaged in the first hydraulic type speed change unit 17′, andthe other hydraulic clutch is disengaged. In the latter case, forexample, when the primary speed change lever 81 is either shifted up orshifted down between the second speed position and the third speedposition, engaged-clutch exchange is performed between the hydraulicclutches 57 and 59 in the first hydraulic type speed change unit 17, andengaged-clutch exchange is performed between the hydraulic clutches 66and 67 in the second hydraulic type speed change unit 20.

[0123] In short, two speed changes can be achieved. One of the speedchanges is achieved such that, in the overall primary speed changemechanism, one hydraulic clutch is disengaged, and one hydraulic clutchis newly engaged (which hereinbelow will be referred to as“speed-changing with one-objective-based hydraulic clutches beingdisengaged/engaged”). The other speed change is achieved such that, inthe overall primary speed change mechanism, two hydraulic clutches aredisengaged, and two hydraulic clutches are newly engaged (whichhereinbelow will be referred to as “speed-changing withtwo-objective-based hydraulic clutches being disengaged/engaged”). Ineither case, it is essential to secure the aforementioned common slipregion.

[0124]FIGS. 17 and 18 each show time-transitional graphs (graphs ofoperating pressures p relative to a time t) regarding individualhydraulic-clutch-operating hydraulic pressures in the first hydraulictype speed change unit 17′ and the second hydraulic type speed changeunit 20 on the same time axis. Concurrently, the figures each showtime-transitional voltage graphs regarding the pressure sensors. FIG. 17shows the speed change with one-objective-based hydraulic clutches beingdisengaged/engaged, in which the primary speed change lever 81 is eithershifted up and shifted down between the first speed position and thesecond speed position. FIG. 18 shows the speed-changing withtwo-objective-based hydraulic clutches being disengaged/engaged, inwhich the primary speed change lever 81 is shifted up and shifted downbetween the second speed position and the third speed position. For eachpressure-rising portion of the individual hydraulic-pressuretime-transitional graphs in FIGS. 17 and 18, thefluid-chamber-filling-out required period (engagement start time t₀ topressure-rising time ta) shown in FIG. 10 is not taken into account, andthe pressure is assumed to increase higher than the piston-holdingpressure pa as soon as the position of the primary speed change lever 81has been shifted. In addition, pressure sensors PSL, PSM, PSH, PS1, PS2,and PS3 shown in each of FIGS. 17 and 18 are each assumed to function asa switch that turns ON when the pressure increases higher than aswitch-sensitive pressure pb which is set higher than the piston-holdingpressure pa.

[0125] Hereinbelow, FIG. 17 will be explained. First, when the primaryspeed change lever 81 is set either to the first speed position or tothe second speed position, in the second hydraulic type speed changeunit 20, a hydraulic pressure 66 p is kept at the normal pressure P1,the pressure sensor PS1 keeps turning ON, individual hydraulic pressures67 p and 68 p for the hydraulic clutches 67 and 68 remain to be 0, andthe pressure sensors PS2 and PS3 are turned OFF.

[0126] In the first hydraulic type speed change unit 17′, when theprimary speed change lever 81 is shifted up from the second speedposition to the first speed position, a hydraulic pressure 59 p for theengagement-objective clutch 59 begins to rise, and is then graduallyincreased to the normal pressure p₁. In this course, when a hydraulicpressure 59 a reaches the switch-shifting pressure pb, the pressuresensors PSH turns ON. Slightly later than the rise in the pressure 59 p,a hydraulic pressure 57 p for the disengagement-objective clutch 57begins to decrease, and a pressure-decrease line portion thereof crossesa pressure-increase line portion of the hydraulic pressure 59 p. Thatis, an operating hydraulic pressure for the disengagement-objectiveclutch decreases in the course of gradual increase in an operatinghydraulic pressure for the engagement-objective clutch. In this way, asshown by the slanting lines, common slip regions of the two hydraulicclutches 57 and 59 are secured. When the decreasing hydraulic pressure57 p is reduced lower than the switch-shifting pressure pb, the pressuresensor PSL in the ON state turns OFF.

[0127] When the primary speed change lever 81 is shifted down from thesecond speed position to the first speed position, the hydraulicpressure 57 p for the hydraulic clutch 57 begins to rise, and thengradually increases; and the hydraulic pressure 59 p for the hydraulicclutch 59 decreases. As described above, common slip regions are securedas in the above case. In the pressure-rising course, when the hydraulicpressure 57 p reaches the switch-shifting pressure pb, the pressuresensor PSL in the OFF state turns ON. When the decreasing hydraulicpressure 59 p is reduced lower than the switch-shifting pressure pb, thepressure sensor PSH in the ON state turns OFF.

[0128] When the state at the shifted-up (from the first speed positionto the second speed position) time is compared to the state at the timeof the shifted-down (from the second speed position to the first speedposition), the common slip region at the shifted-down time is relativelynarrow. At the shifted-down time, since rotational inertia prior to theshifting (in the state of the second speed position) is imposed as atransmission force on a rotation shaft on the secondary side of theclutch, which is engaged/disengaged, the slip regions are controlled tobe narrow to allow the speed to be changed smoothly and quickly.

[0129] In FIG. 18 showing a case where the speed is changed between thesecond speed position and the third speed position, hydraulic clutchesare disengaged/engaged in each of the first hydraulic type speed changeunit 17′ and 20. Specifically, in the first hydraulic type speed changeunit 17′, hydraulic-pressure control as described referring to FIG. 17is performed for disengagement/engagement operation of the hydraulicclutches 57 and 59 individually at a time of shifting-up (from thesecond speed position to the third speed position) and at a time ofshifting-down (from the third speed position to the second speedposition). On the other hand, in the second hydraulic type speed changeunit 20, simultaneously with rise in the hydraulic pressure 57 p for thehydraulic clutch 57, the engagement-objective clutch 67 p begins torise, and then gradually increases parallel to the increase in thehydraulic pressure 57 p up to the normal pressure p₁. Simultaneously, inparallel to reduction in the hydraulic pressure 59 p for thedisengagement-objective clutch 59 in the first hydraulic type speedchange unit 17′, and slightly later than rise in the hydraulic pressure67 p, the hydraulic pressure 66 p for the disengagement-objective clutch66 decreases. At the shifting-down time, the hydraulic pressure 66 p forthe engagement-objective clutch 66 increases, and the hydraulic pressure67 p for the disengagement-objective clutch decreases in synchronizationwith the increase in the hydraulic pressure 59 p for theengagement-objective clutch 59 and the reduction in the hydraulicpressure 57 p for the disengagement-objective clutch 57. In this way,similarly to the time-transitional hydraulic pressure property in thefirst hydraulic type speed change unit 17′, which is shown in FIG. 17, asignificantly large common slip region is secured at the shifting-uptime in either the first hydraulic type speed change unit 17′ or thesecond hydraulic type speed change unit 20. ON/OFF operations of thepressure sensors for the individual hydraulic clutches are disclosed.

[0130]FIGS. 17 and 18 are adaptive to the primary speed change mechanism1′. To arrange them to be adaptive to the primary speed change mechanism1, modification is made such that, in the case of FIG. 17, the operatinghydraulic pressure 59 p for the high-speed-use hydraulic clutch 59 isreplaced with the operating hydraulic pressure 58 p for theintermediate-speed-use intermediate-speed clutch 58 in the firsthydraulic type speed change unit 17, and the hydraulic clutch 59 iscontrolled to transit at substantially in all periods. In addition, thetime-transitional voltage graph regarding the pressure sensors PSH isreplaced with the time-transitional voltage graph regarding the pressuresensor PSM, and the pressure sensors PSH is controlled to be in the OFFstate in all periods.

[0131]FIG. 18 shows the case of the speed-change withtwo-objective-based hydraulic clutches being disengaged/engaged. In theprimary speed change mechanism 1, since the change is performed bysimply exchanging between the disengagement/engagement operations of theintermediate-speed clutches 58 and 59 only in the first hydraulic typespeed change unit 17, it is not applicable. To arrange the case of FIG.18 to be adaptive to the primary speed change mechanism 1, modificationis made such that, for example, the primary speed change lever 81 isshifted between the third speed position and the fourth speed position.In this case, at the shifting-up time in the first hydraulic type speedchange unit 17, the hydraulic clutch 59 is controlled to disengage, andthe hydraulic clutch 57 is controlled to engage. Therefore, thetime-transitional graphs regarding the hydraulic pressures 57 p and 59p, and the time-transitional voltage graphs regarding the pressuresensors PSL and PSH, which are shown in FIG. 18, can be used withoutmodification; and the intermediate-speed clutch 58 transits atsubstantially 0 in all periods, and the pressure sensor PSM transits inthe OFF state in all periods. In the second hydraulic type speed changeunit 20, the disengagement-objective clutches 66 and 67 are controlledto disengage and engage, and the hydraulic clutch 68 is controlled toremain in a disengaged state. Therefore, the hydraulic-pressuretime-transitional graphs and the pressure-sensor time-transitionalvoltage graphs may be used without modification.

[0132] The above describes that, in the cases shown in FIGS. 17 and 18,the fluid-chamber-filling-out required periods for engagement-objectiveclutches between the hydraulic clutches 57 and 59 are not taken intoaccount for the convenience of description. In practice, however, thehydraulic-pressure time-transitional graphs take the forms, as shown inFIG. 10, which have the fluid-chamber-filling-out required periods. Thefluid-chamber-filling-out required period varies depending on whetherthe hydraulic clutches are disengaged/engaged at the time ofspeed-changing with the one-objective-based clutches or thetwo-objective-base clutches. Specifically, the fluid-chamber-filling-outrequired period increases substantially twice as much, compared to thecase of the single target set. In addition, the required period variesdepending on the engine revolution frequency. Specifically,proportionally to the reduction in the engine revolution frequency,driving forces of the hydraulic pumps are reduced. Therefore, hydraulicpressures are slowly increased to increase the fluid-chamber-filling-outrequired period.

[0133] Suppose a delay time of a disengagement start time ts withrespect to the engagement start time t₀ is fixedly set to obtain asuitable common slip region corresponding to the operation for causing aone-objective-based hydraulic clutches to disengage/engage. In thiscase, when speed-changing with one-objective-based hydraulic clutchesbeing disengaged/engaged is performed, the pressure-rising time ta isdelayed greater than that in the former case to thereby relativelyreduce the delay time of the disengagement start time ts with respect tothe pressure-rising time ta. Therefore, the common slip region isnarrower than the common slip region that can be obtained in thespeed-change pattern for causing speed-changing with theone-objective-based hydraulic clutches being disengaged/engaged. Thatis, the area of the common slip region is small to thereby impair thespeed-change feeling. Depending on the case, the disengagement starttime ts can be earlier than the pressure-rising time ts to therebydisable a common slip region to be produced (that is, after adisengagement-objective clutch is disengaged-away from a slip state, thehydraulic pressure for an engagement-objective clutch rises to cause itto be in a slip state). The incident of this kind does not conform tothe above-described basic concept.

[0134] The present invention is therefore made to compensate for thedifference in the clutch-fluid-chamber-filling-out required periods inthe two cases. To perform the compensation, the invention allows delaytimes of the disengagement start time ts with respect to the engagementstart time t₀ to be set differently corresponding to the individualcases.

[0135] For delay times of the disengagement start time ts with respectto the engagement start time t₀, as shown in FIG. 3 or 4, a certainnumber of delay patterns is stored in the time-setting circuit 90.According to an input signal from a member such as the potentiometer 82or the tachometer 83, the logical circuit 80 outputs adelay-pattern-selection parameter to the time-setting circuit 90. Asolenoid control signal based on a selected delay pattern is input fromthe time-setting circuit 90 to the delay circuit 88. The delay circuit88 is used to delay an OFF-drive time of a solenoid provided to thesolenoid-driver circuit 86 by a predetermined amount, thereby allowingthe delay time to be obtained.

[0136] In addition, corresponding to the total of four patterns at theshifting-up time and the shifting-down time in a rated-revolution stateand a low-speed-revolution state of the engine, the aforementioned delaytimes are set, and concurrently, a pressure-decrease property fordisengagement-objective clutches is set.

[0137] Specifically, the hydraulic-pressure control patterns for theprimary-speed-change hydraulic clutches are provided corresponding tofour divisional cases at the shifting-up time and the shifting-down timein the rated revolution state and the low-speed revolution state of theengine.

[0138] Hereinbelow, a description will be made regarding an embodimentshown in FIGS. 19 to 22, an embodiment shown in FIGS. 23 and 24, and anembodiment shown in FIGS. 25 and 26. In each of the figures, A shows ahydraulic-pressure control graph in the case of speed-changing withone-objective-based hydraulic clutches being disengaged/engaged, and Bshows a hydraulic-pressure control graph in the case of speed-changingwith two-objective-based hydraulic clutches being disengaged/engaged. Ineach of the figures, A and B are the same in unit-time intervals on thehorizontal axis and unit-pressure intervals on the vertical axis.

[0139]FIG. 19 shows hydraulic-pressure control states at a shifting-uptime in the rated-speed engine revolution state. The tachometer 83 shownin FIG. 3 or 4 inputs a signal representative of the rated revolutionstate of the engine to the logical circuit 80. The potentiometer 82inputs to the logical circuit 80 a signal that represents the positionof the primary speed change lever 81 before or after a shifting-upoperation. The logical circuit 80 determines whether the case requiresonly one hydraulic clutch to be newly engaged or requires two hydraulicclutches to be newly engaged.

[0140]FIG. 19A shows a hydraulic-pressure control state in the casewhere one-objective-based hydraulic clutches are disengaged/engaged.Therefore, at a disengagement start time ts, while one hydraulic clutchis disengaged to thereby reduce an operating pressure p therefor,another hydraulic clutch remains engaged at an operating pressure pbeing kept at a normal pa. FIG. 19B shows a control state in the casewhere two-objective-based hydraulic clutches are disengaged/engaged.Therefore, at the disengagement start time ts, while two hydraulicclutches are disengaged to thereby reduce operating pressures p for thetwo clutches. In either one of the case of FIG. 19A or the case of FIG.19B, a solenoid-excitation-relief control pattern for producing thehydraulic-pressure-decrease property graph D1 is selected in thepressure-setting circuit 89, and the operating pressure p for thehydraulic clutch to be disengaged is abruptly reduced to a level equalto or lower than the piston-holding pressure pa.

[0141] The arrangement is modified such that engine load states in thecases shown in A and B of FIG. 19 are not different from each other, andthe same solenoid-exciting pattern is set in the pressure-settingcircuit 87 for the both to implement the same pressure-increase pattern.This arrangement is common to cases shown in FIGS. 20 to 22, which willbe described below.

[0142] As can be seen through the comparison between FIGS. 19A and 19B,a delay time Δt2 of the disengagement start time ts from an engagementstart time t₀ (which hereinbelow will be referred to as a “delay time”),which is shown in FIG. 19B, in the case where two-objective-basedhydraulic clutches being disengaged/engaged is set longer than a delaytime Δt1 in the case where one-objective-based hydraulic clutches aredisengaged/engaged corresponding to a time difference from theengagement start time t₀ to a pressure-rising time ta. Thereby, thestates and the areas of common slip regions shown by slanting lines in Aand B are controlled to be substantially the same. Therefore, even whenany type of shifting-up is performed, good speed-change feelings thatare similar to each other can be obtained.

[0143]FIG. 20 shows hydraulic-pressure control states at a shift-downtime in a rated-engine-revolution condition, in which A shows a casewhere one-objective-based hydraulic clutches are disengaged/engaged, andB shows a case where two-objective-based hydraulic clutches aredisengaged/engaged. Similarly to the cases shown in FIG. 19, in eitherof the cases shown in A and B, a pressure-decrease pattern is set to D1.

[0144] Similarly to the cases in FIG. 19, to obtain substantially thesame common slip region in A and B with regard to the state and thearea, a delay time Δt2′ in B is controlled to be longer than a delaytime Δt1′ in A, corresponding to the time difference from an engagementstart time t₀ to a pressure-rising time ta.

[0145] However, taking inertia generated at a time of vehiculartraveling into account, Δt1′ and Δt2′ at shifting-down times arecontrolled to be shorter than Δt1 and Δt2, respectively. Thereby, thecommon slip region is reduced narrower than that at the shifting-up timeshown in FIG. 19 to thereby implement improvement in energy efficiency.

[0146]FIG. 21 shows hydraulic-pressure control states at a shifting-uptime in a low-speed (idle revolution speed, or a revolution speedsimilar thereto) engine revolution. Therefore, the tachometer 83 shownin FIG. 3 or 4 inputs a signal representative of the low-speed enginerevolution state to the logical circuit 80. Setting of delay times Δt1and Δt2 is performed similar to that shown in FIG. 6. However, at thelow-speed engine revolution state, the revolution frequency of thehydraulic pump 50 is reduced lower than that at the rated-speed enginerevolution time. Therefore, the clutch fluid-chamber-filling-outrequired period required for rise in a hydraulic pressure p to apressure equal to or higher than the normal pressure pa, that is, thetime from an engagement start time t₀ to a pressure-rising time ta islonger than that that at each of the rated-speed engine revolutionstates shown in FIG. 19. Therefore, the delay times Δt1 and Δt2, whichhave been set taking the fluid-chamber-filling-out required period inthe rated-speed engine revolution state into account, are used withoutmodification. Concurrently, similarly to the case shown in FIG. 19, apressure-decrease property graph is set to D1. In this case, in either Aor B, a common slip region is very narrow. That is, since a sufficientarea cannot be secured, the speed-change feeling is impaired.

[0147] In the case shown in FIG. 21, a solenoid-excitation-relievingpattern is selected in the pressure-setting circuit 89 to slowly reducean operating pressure for a hydraulic clutch, that is, to implement thehydraulic-pressure-decrease pattern D2 or D3. To cause the pressurereduction to be slow as described above, the variable aperture Va ineach of the electromagnetic proportion selector valves is used.

[0148] According to the pressure control graph in FIG. 21A showing thecase of a time of shifting-up carried out in a state whereone-objective-based hydraulic clutches are disengaged/engaged, thepressure is abruptly reduced at a disengagement start time. After thepressure is reduced to a specific level of the pressure, thepressure-decrease pattern D2 in which the pressure slowly decreases isset. Thereby, the area of a common slip region is controlled to besubstantially the same as that shown in FIG. 19.

[0149] In addition, according to the pressure control graph in FIG. 21Bshowing the case of a time of shifting-up carried out in a state wheretwo-objective-based hydraulic clutches are disengaged/engaged, theabove-described fluid-chamber-filling-out required period is so longthat even a disengagement start time ts set according to a delay timeΔt2 set longer than the pressure-rising time ta is earlier than thepressure-rising time ta. Therefore, the hydraulic-pressure-decreasepattern D3 in which the pressure is reduced even slower than that in thepressure-decrease pattern D2 is selected to obtain a common slip region.In addition, the area of the region is controlled to be substantiallythe same as that shown in FIG. 19.

[0150]FIG. 22 shows hydraulic-pressure control states at a shifting-downtime in a low-speed engine revolution. Similarly to the case shown inFIG. 21, the tachometer 83 shown in FIG. 3 or 4 inputs a signalrepresentative of the low-speed engine revolution state to the logicalcircuit 80. In A and B of FIG. 22, the graphs in A and B of FIG. 21,i.e., the same pressure-decrease patterns as those in the shifting-uptimes, are set in the pressure-setting circuit 89. In the time-settingcircuit 90, similarly to the cases at the times of shifting-down carriedout in the rated-speed engine revolution state, which are shown in FIG.20, to improve the energy efficiency, delay times Δt1′ and Δt2′ that arerespectively shorter than the delay times Δt1 and Δt2 at the shifting-uptime are set. In this way, common slip regions having substantially thesame areas of the common slip regions shown in FIGS. 20A and 20B aresecured to thereby allow good speed-change feeling to be obtained.

[0151] The present embodiment has a method to obtain a constant commonslip region regardless of the time difference in thefluid-chamber-filling-out required periods of an engagement-objectiveclutch between the rated-speed engine revolution state and the low-speedengine revolution state. As can be seen through the comparison betweenFIGS. 19A and 21A, between FIGS. 19B and 21B, between FIGS. 20A and 22A,or between FIGS. 20B and 22B, to achieve the aforementioned method, theembodiment modifies the pressure-decrease property for thedisengagement-objective clutch. As an alternative method, it isconceivable that the delay time, i.e., the time between the engagementstart time t₀ and the disengagement start time, is modified. Inaddition, it is conceivable that both the pressure-decrease property anddelay time are modified.

[0152] Hereinbelow, the hydraulic-pressure control illustrated in FIGS.23 and 24 will be described. The hydraulic-pressure control shown inFIGS. 19 to 22 controls the delay time to be different in the case whereone-objective-based hydraulic clutches are disengaged/engaged and thecase where two-objective-based hydraulic clutches aredisengaged/engaged. That is, in the latter case, the delay time is setto the delay time Δt2 or Δt2′. Hydraulic-pressure control shown in FIG.23 or 24, however, uses delay time Δt1 or Δt1′ set either in the case ofthe speed-changing with one-objective-based hydraulic clutches beingdisengaged/engaged or in the case of the speed-changing withtwo-objective-based hydraulic clutches being disengaged/engaged.Thereby, instead of controlling the delay time to be different, thehydraulic-pressure control controls the pressure-decrease pattern to bedifferent in the individual cases.

[0153]FIG. 23 shows cases of shifting-up carried out in the rated-speedengine revolution state. The delay time from an engagement start time t₀to a disengagement start time ts is set to Δt1 in either the case whereone-objective-based hydraulic clutches are disengaged/engaged or thecase where two-objective-based hydraulic clutches aredisengaged/engaged. In each of the cases, asolenoid-excitation-relieving pattern suitable to the above arrangementis selected in the pressure-setting circuit 89. Thereby, as shown inFIG. 23A, in the former case, control as represented by thehydraulic-pressure-decrease property graph D1 as in the case of FIG. 19Acan be implemented; and as shown in 23B, in the latter case, control asrepresented by the pressure-decrease graph D2 including the portionwhere the pressure is slowly reduced can be implemented. Since the delaytime is set to Δt1, when two-objective-based hydraulic clutches aredisengaged/engaged, compared to the case where one-objective-basedhydraulic clutches are disengaged/engaged, a fluid-chamber-filling-outperiod (t₀ to ta) is increased to be relatively long, and the timebetween the hydraulic-pressure-rising time ta and the disengagementstart time ts is reduced relatively short. However, the pressure isinstead slowly reduced, thereby controlling the areas of the common slipregions in the cases of A and B to be substantially the same.

[0154]FIG. 24 shows cases of shifting-down carried out in therated-speed engine revolution state. The delay time is set to Δt1′,which is shorter than Δt1, in either one of the case whereone-objective-based hydraulic clutches are disengaged/engaged or wheretwo-objective-based hydraulic clutches are disengaged/engaged. In eachof the cases, a solenoid-excitation-relieving pattern suitable to thisarrangement is selected in the pressure-setting circuit 89. Thereby, asshown in FIG. 24A, in the former case, control as represented by thehydraulic-pressure-decrease property pattern D1 as in the case of FIGS.20A (23A) can be implemented; and as shown in 24B, in the latter case,control as represented by the pressure-decrease graph D2 including theportion where the pressure is slowly reduced can be implemented as inthe case of FIG. 23B. Thereby, the common slip regions shown in FIGS.24A and 24B, which improves the energy efficiency, are controllednarrower than the common slip regions shown in FIGS. 23A and 23B.However, the areas of the common slip regions in the two cases arecontrolled to be substantially the same, as shown in FIGS. 24A and 24B.

[0155] The embodiment shown in FIGS. 23A and 23B discloses the controlin the rated-speed engine revolution state. However, control similarthereto can be implemented even in the low-speed engine revolutionstate. In this case, the delay time is controlled to be same ineither-one of the cases where one-objective-based hydraulic clutches aredisengaged/engaged or where two-objective-based hydraulic clutches aredisengaged/engaged. In addition, the pressure-decrease pattern is set toallow the provision of common slip regions having the same areas as thecommon slip regions that can be obtained in the low-speed enginerevolution state. However, for example, at a shifting-up time whentwo-objective-based hydraulic clutches are disengaged/engaged, as shownin FIG. 22B, the fluid-chamber-filling-out period (t₀ to ta) is so longthat the common slip regions may not be obtained with the delay timeΔt1′ being set. In this case, to allow the implementation of control asrepresented by a pressure-decrease graph including a pressure-decreaseslanting portion further gentler than D3 shown in FIG. 22B, it isconceivable that a solenoid-excitation-relieving pattern correspondingthereto is stored in the pressure-setting circuit 89 or that the delaytime is set longer than Δt1′ (for example, it is set to Δt2′) only forthe particular case.

[0156]FIG. 25 shows an embodiment of control at a time of shifting-upcarried out in the rated-speed engine revolution state without a delaytime being provided. Specifically, a disengagement start time ts iscontrolled to match an engagement start time t₀. Because of thisarrangement, a solenoid-excitation-relieving pattern suitable to thisarrangement is selected in the pressure-setting circuit 89. Thereby, inthe case shown in A, control that can be represented by apressure-decrease graph (such as D2) including a portion representing aslow reduction in the pressure is implemented; and in the case shown inB, taking the fluid-chamber filling-out time is longer than that in thecase of A into account, control that can be represented by apressure-decrease graph (such as D3) including a portion representing aneven-slower reduction in the pressure is implemented. According to theabove, although no delay time is provided, common slip regions havingsubstantially the same areas as those in the cases shown in FIGS. 19Aand 19B can still be secured.

[0157]FIGS. 26A and 26B show cases at a time of shifting-up carried outin the rated-speed engine revolution state, in which delay times areindividually set shorter than the delay times Δt1 and Δt2 set in thecases shown in FIGS. 19A and 19B. In each of the cases, asolenoid-excitation-relieving pattern suitable to this arrangement isselected in the pressure-setting circuit 89. Thereby, in the case shownin A, control that can be represented by a pressure-decrease graph (suchas D2) including a portion representing a slow reduction in the pressureis implemented; and in the case shown in B, taking the fluid-chamberfilling-out time is longer than that in-the case of A into account,control that can be represented by a pressure-decrease graph (such asD3) including a portion representing an even-slower reduction in thepressure can be implemented. Thus, the delay time is reduced, andconcurrently, a pressure-decrease property is appropriately set.Thereby, common slip regions having substantially the same areas asthose in the cases shown in FIGS. 19A and 19B can be secured, and goodoperational feeling can be obtained.

[0158] Although the embodiments of the control shown in FIG. 25 or 26 donot disclose a case other than that at the time of shifting-up carriedout in the rated-speed engine revolution state, each of them may also beapplied to the case at a time of either shifting-up or shifting-down ina low-speed engine revolution state. In this case, the arrangement maybe made such that, while a delay time is not provided, or a delay timeis reduced, pressure-decrease properties (pressure-decrease graphs) areappropriately set to allow common slip region to be secured.Alternatively, the hydraulic-pressure control methods shown in FIGS. 19to 26 may be combined corresponding to various cases. For example, themethod may be arranged such that no delay time is provided at the timeof shifting-up in the rated-speed engine revolution state; or a shortdelay time is provided at the time of shifting-down in the rated-speedengine revolution state.

[0159] In addition, the control method may be arranged such that, as inthe case of each of the individual embodiments shown in FIGS. 19 to 22,revolution states of the engine are classified into the rated-speedrevolution state and the low-speed revolution state to varyhydraulic-pressure-decrease properties for hydraulic clutches.Alternatively, the control method may be arranged such that the enginerevolution states are not classified, but the slant degree of thehydraulic-pressure-decrease graph as shown in FIG. 11 can be variedcontinuously so as to correspond to the revolution frequencies of theengine 10, that is, so as to be less in proportion to the reduction inthe revolution frequency. For example, the method may be arranged suchthat a detection value obtained by the tachometer 83 is compared to therated engine revolution frequency, and the aperture of the variableaperture Va can be controlled corresponding to the comparison result sothat control which can be represented by a pressure-decrease graph ofwhich the slant degrees are less in proportion to the reduction in theengine revolution frequency can be implemented in the pressure-settingcircuit 89. According to this arrangement, a fluid-filling-out time fora hydraulic clutch, which is required to be longer in proportion to thereduction in the engine revolution frequency, can be continuouslycompensated for corresponding to the engine revolution frequency, and acommon slip region can be secured in the hydraulic-pressure controlgraph, thereby allowing good speed-change operation can obtained at alltimes.

[0160] Hereinbelow, a description will be made regarding detection of anabnormal hydraulic clutch, the detection being performed using thepressure sensor provided between each of the electromagnetic proportionselector valves and hydraulic clutch corresponding thereto, andregarding hydraulic-pressure control according to the detection. Apressure-increase property for an engagement-objective clutch, apressure-decrease property for a disengagement-objective clutch, andstarting times of the engagement and disengagement courses arespecifically set corresponding to required conditions. This allows theprediction to be made for time when a pressure-sensor-detecting value ofhydraulic pressure for a disengagement-objective clutch begins todecrease up to a pressure value corresponding to tolerable absorptionenergy value of a lining of a friction disc of the hydraulic clutch(switch-shifting pressure pb shown in the above-described figures suchas FIGS. 17 and 18). Therefore, time is set through the prediction ofthe pressure-decrease time. When the pressure sensors indicates a levelhigher than the aforementioned pressure value (switch-shifting pressurepb) even after the set time has passed, the logical circuit 80 receivesan input signal therefrom and thereby determines that thedisengagement-objective clutch is abnormal because of, for example,entrance of foreign substances.

[0161] As shown in the figures such as FIGS. 17 and 18, each of thepressure sensors is configured to function as a switch that turns ONwhen pressure is higher than the switch-shifting pressure pb. In thiscase, when one of the pressure sensors for a disengagement-objectiveclutch still remains in the ON state even after the above-described settime has passed, it determines the clutch to be abnormal.

[0162] For the pressure sensor for the disengagement-objective clutch toperform the abnormality detection, it needs to identify adisengagement-objective clutch that is variable according to varioustypes of speed changes. Therefore, control therefor is complicated. Ineach of the first hydraulic type speed change unit 17 (17′) and thesecond hydraulic type speed change unit 20, a disengagement-objectiveclutch is supposed to be alternatively selected at a speed-change time.Therefore, in each of the hydraulic type speed change units, when two ormore pressure sensors are in a state higher than the switch-shiftingpressure pb (or, they are turned ON), determination can be made that thehydraulic type speed change unit includes an abnormal hydraulic clutch.Specifically, according to a calculation performed for the number ofpressure sensors that have detected hydraulic pressures higher than theswitch-shifting pressure pb after the above-described set time haspassed at a speed-change time in each of the individual hydraulic typespeed change units, determination can be made whether the hydraulic typespeed change unit is normal or abnormal without performingidentification of disengagement-objective clutches. This method may beemployed as an abnormality-determining method.

[0163] As described above, when an abnormal clutch is detected,engagement commands for the solenoids for all the electromagneticproportion selector valves are reset by the logical circuit 80 and thesolenoid-driver circuits 85 and 86. That is, even hydraulic clutchescommanded to engage are disengaged. The unit is thus controlled to be ina state where at most only a hydraulic clutch that cannot be disengagedbecause of a foreign substance intruded into the fluid chamber thereofis filled with operating fluid at a pressure higher than theswitch-shifting pressure. Thereby, abnormal double engagement in a geartrain is prevented.

[0164] Alternatively, it is conceivable to increase an operatingpressure p that is applied to a hydraulic clutch which is to beconnected to an pressure sensor issuing an ON signal positively at theearliest time, that is, a hydraulic clutch engaged before shifting andincluded a foreign substance in itself to the normal pressure p₁. Inthis way, the aforementioned hydraulic clutch is controlled to be in acompletely-press-contacted state. Thereby, at least the foreignsubstance included in the fluid chamber of the hydraulic clutch is notsandwiched by the clutch in its engagement, and is kept in a state offloating in the fluid, thereby allowing the hydraulic clutch to beprevented from damage.

[0165] In any one of the hydraulic-pressure control methods, it ispreferable that the existence of an abnormal clutch is notified to anoperator as a result of the abnormal-clutch detection in a way of, forexample, lighting a warning lamp.

[0166]FIG. 27 shows a flowchart for control according to an embodiment.The control is implemented for an instance in which a foreign substanceis carried into a hydraulic clutch. In the control, the pressure sensoris assumed to have a function as a switch that turns ON in response tothe detection of a hydraulic pressure higher than the switch-shiftingpressure pb. First, in step 401, processing determines whether or notthe engine 10 is in operation. If the engine 10 is in operation,processing proceeds to step 402. In step 402, if, in three pressuresensors in the first hydraulic type speed change unit 17 or in twopressure sensors in the first hydraulic type speed change unit 17′, twoor more pieces thereof are turned ON, processing proceeds to step 403.In step 403, in order to increase the pressure for a hydraulic clutchcorresponding to a pressure sensor that has been in an ON state beforethe primary speed change lever 81 was shifted, a solenoid for anelectromagnetic proportion selector valve is excited, and othersolenoids are relieved from excitation to control the hydraulic clutchesother than the aforementioned hydraulic clutch to disengage. In short,the state of the first hydraulic type speed change unit 17 (17′) isreturned to the pre-shift state. Alternatively, the processing in thestep may be modified such that solenoids for all the electromagneticproportion selector valves in the first hydraulic type speed change unit17 (17′) are relieved from excitation. Subsequently, in step 404, awarning means (a lamp or a buzzer) for notifying abnormality caused inthe first hydraulic type speed change unit 17 (17′) is operated.

[0167] Subsequently, at step 405, in the three pressure sensors in thesecond hydraulic type speed change unit 20, when two or more piecesthereof are turned ON, the control in steps 403 and 404 is performed.Also in the second hydraulic type speed change unit 20, if two or morepressure sensors are not turned ON, processing determines the statethereof to be free of abnormality that disables the engagement of ahydraulic clutch, allows an engagement-objective hydraulic clutch toengage, and allows a disengagement-objective hydraulic clutch todisengage (step 406).

[0168] The above almost completes intended description regarding thehydraulic-clutch hydraulic-pressure control of the present invention.Hereinbelow, a description will be made regarding an embodiment of ahydraulic circuit shown in FIG. 28, which is configured by modifying thehydraulic circuit shown in FIG. 2 to be more primitive. Theabove-described electromagnetic proportion selector valves VL, VM, VH,V1, V2, and V3 are replaced by electromagnetic selector valves VAL, VAM,VAH, VA1, VA2, and VA3, respectively. The fluid-feeder circuit 70 isconnected to these electromagnetic selector valves individually viaelectromagnetic proportion valves 110. A tank port of each of theelectromagnetic selector valves is connected to an electromagneticcontroller valve 111. Each of the electromagnetic controller valves 111is equipped with a variable aperture 111 a. In FIG. 28, the variableaperture 111 a is shown outside of the electromagnetic controller valve111 to be easily viewed. The variable apertures 111 a are providedinstead of the variable apertures Va shown in FIG. 2.

[0169] Each of the electromagnetic proportion valves 110 is set to aneutral position N with a corresponding solenoid being set to anonexcitation state while it is set to an operating position I with acorresponding solenoid excited. To cause a hydraulic clutch to bedisengaged, the electromagnetic proportion valve 110 correspondingthereto is set the neutral position N, thereby discontinuing theconnection between the electromagnetic selector valve, which isconnected to the clutch, and the fluid-feeder circuit 70. Concurrently,a solenoid for the electromagnetic selector valve is relieved from theexcitation, and is set to a fluid tank via the correspondingelectromagnetic controller valve 111. At this time, with theelectromagnetic controller valve 111 being set to an X position, fluidfed from the electromagnetic controller valve 111 is returned to thefluid tank without the variable aperture 111 a being used therebetween.Therefore, there is implemented the vertically-linear hydraulic-clutchpressure reduction at the disengagement start time ts, which is shown inFIG. 11. With the electromagnetic controller valve 111 being set to a Yposition, operating fluid is gradually returned to the fluid tank viathe variable aperture 111 a, thereby slowly reducing thehydraulic-pressure pressure. Therefore, to perform control asrepresented by the hydraulic-pressure-decrease property graph D1 shownin FIG. 11, when each of the electromagnetic proportion valves 110 andthe electromagnetic proportion valves is set to a fluid-returningposition (neutral position N), the electromagnetic controller valve 111is set to the X position throughout the entire reduction course throughwhich the pressure p in the fluid chamber of the disengagement-objectiveclutch is reduced substantially to 0. Similarly, to perform control asrepresented by either the pressure-decrease graph D2 or D3, when each ofthe electromagnetic proportion valves 110 and the electromagneticproportion valves is similarly set to a fluid-returning position(neutral position N), the electromagnetic controller valve 111 is firstset to the X position to abruptly reduce the pressure p in the fluidchamber of the disengagement-objective clutch, the electromagneticcontroller valve 111 is then switched to be set to the Y position tothereby slowly reduce the operating pressure p. In addition, theaperture of the variable aperture 111 a is adjusted to select one of thecontrol patterns represented by D2 and D3.

[0170] To engage a hydraulic clutch, the solenoid for the correspondingelectromagnetic proportion valve 110 is excited to be set to a positionI, and a solenoid for an electromagnetic selector valve to be connectedthereto is also excited to control the unit to be in a state where fluidis fed from the fluid-feeder circuit 70 to the intended hydraulicclutch. In this state, the electromagnetic proportion valve 110 iscontrolled to reduce the aperture for the fluid that is fed from thefluid-feeder circuit 70 to the electromagnetic selector valve, therebyincreasing the operating pressure p that is applied to theengagement-objective clutch.

[0171] The hydraulic-circuit configuration shown in FIG. 28 can also becombined with the electrical controller circuit shown in FIG. 3 or 4 tobe used for the hydraulic-pressure control as illustrated in FIGS. 17 to27.

INDUSTRIAL APPLICABILITY

[0172] As described above, the present invention functions in the speedchange mechanism having the hydraulic clutches; particularly, itfunctions in the speed change mechanism configured of the plurality ofhydraulic type speed change units connected in tandem, the hydraulictype speed change unit having the plurality of hydraulic clutches thatare alternatively engaged. The invention enables smooth, secure, andcomfortable speed-changing to be implemented at all times regardless ofthe engine revolution frequency and the speed-step shift condition. Inaddition, the invention allows double transmission to be effectivelyavoided at the time of abnormality, such as the entrance of a foreignsubstance in the hydraulic clutch during the speed-changing. Therefore,the invention provides significant advantages for vehicles that employthe invention, such as agricultural tractors and other work tractorsthat require many speed-change steps.

What is claimed is:
 1. A method of performing hydraulic-pressure control in a speed change mechanism comprising a plurality of speed-changing hydraulic clutches, each of which is engaged according to hydraulic-pressure-increase effects and is disengaged according to hydraulic-pressure-decrease effects, wherein, during a speed-changing operation such as to disengage one clutch from an engaged state and to engage another from an disengaged state, an operating hydraulic pressure for the clutch to be engaged from the disengaged state is gradually increased in a time transition, and an operating hydraulic pressure for the clutch to be disengaged from the engaged state is reduced during the gradual pressure increase.
 2. The method of performing hydraulic-pressure control according to claim 1, wherein a specific time-transitional pressure region where an engagement-objective clutch and a disengagement-objective clutch commonly slip at the time of speed-changing operation is secured.
 3. The method of performing hydraulic-pressure control according to claim 1, wherein, at the time of speed-changing operation, an operating-hydraulic-pressure in the disengagement-objective clutch starts decreasing after a piston-holding pressure arises in the engagement-objective clutch by filling fluid in a fluid chamber of the engagement-objective clutch.
 4. The method of performing hydraulic-pressure control according to claim 1, wherein said hydraulic-pressure control is performed according to control of an electromagnetic pressure proportion valve provided for each of the plurality of speed-changing hydraulic clutches.
 5. The method of performing hydraulic-pressure control according to claim 1, wherein, at the time of speed-changing operation, at least either a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch or a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary corresponding to engine revolution frequencies.
 6. The method of performing hydraulic-pressure control according to claim 5, wherein, when the time difference is controlled to vary, the time difference is set longer in proportion to reduction in the engine revolution frequency or in a case where the engine revolution frequency is equal to or lower than a specific level.
 7. The method of performing hydraulic-pressure control according to claim 5, wherein, when the time-transitional decrease property is controlled to vary, the time-transitional decrease property is set to decrease slower in proportion to reduction in the engine revolution frequency or in a case where the engine revolution frequency is equal to or lower than a specific level.
 8. The method of performing hydraulic-pressure control according to claim 5, wherein, during speed-changing, regardless of variations in the time difference or the time-transitional decrease property, the operating-hydraulic-pressure in the disengagement-objective clutch starts decreasing after the piston-holding pressure arises in the engagement-objective clutch by filling fluid in the fluid chamber of the engagement-objective clutch.
 9. The method of performing hydraulic-pressure control according to claim 5, wherein, regardless of variations in the engine revolution frequency, the time-transitional pressure region where the engagement-objective clutch and the disengagement-objective clutch commonly slip at the time of speed-changing operation is maintained substantially to be constant according to the variations in the time difference and the time-transitional decrease property.
 10. The method of performing hydraulic-pressure control according to claim 1, wherein, at the time of speed-changing operation, at least either a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch or a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary depending on whether the speed-changing operation is a shifting-up operation or a shifting-down operation.
 11. The method of performing hydraulic-pressure control according to claim 10, wherein, when the time difference is controlled to vary, the time difference at the time of the shifting-down operation is set shorter than the time difference at the time of the shifting-up operation.
 12. The method of performing hydraulic-pressure control according to claim 5, wherein, during speed-changing, regardless of variations in the time difference and the time-transitional decrease property, an operating-hydraulic-pressure in the disengagement-objective clutch starts decreasing after a piston-holding pressure arises in the engagement-objective clutch by filling fluid in a fluid chamber of the engagement-objective clutch.
 13. The method of performing hydraulic-pressure control according to claim 10, wherein, when the speed-changing operation is the shifting-down operation, the time-transitional pressure region where the engagement-objective clutch and the disengagement-objective clutch commonly slip is reduced narrower than that in the case of the shifting-up operation.
 14. The method of performing hydraulic-pressure control according to claim 1, wherein pressure-detecting means is provided to detect an operating hydraulic pressure for each of the hydraulic clutches, and when the number of said pressure-detecting means for detecting hydraulic pressures higher than a specific pressure value is greater than the number of the hydraulic clutches to be engaged at the time of speed-changing operation, one of two hydraulic-pressure control steps is performed, one hydraulic-pressure control step being performed to engage only those of the hydraulic clutches which have immediate-previously been disengaged, and the other one hydraulic-pressure control step being performed to disengage all the hydraulic clutches.
 15. The method of performing hydraulic-pressure control according to claim 14, switches being provided as said pressure-detecting means, wherein each of said switches turns ON or OFF with respect to the border of the specific pressure value to thereby determine whether the operating hydraulic pressure of corresponding one of the hydraulic clutches is higher or lower than the specific value.
 16. The method of performing hydraulic-pressure control according to claim 1, wherein tractional-load detecting means is provided in a vehicle employing the speed change mechanism, wherein said method modifies at least either a time-transitional increase property in the operating pressure for the hydraulic clutch to be engaged at the time of speed-changing or a time-transitional decrease property in the operating pressure for the hydraulic clutch to be disengaged at the time of speed-changing depending on whether or not said tractional-load detecting means detects a tractional load.
 17. The method of performing hydraulic-pressure control according to claim 1, wherein a governor mechanism capable of performing control of an engine revolution frequency according to detection of an engine load is provided in a vehicle employing said speed change mechanism, wherein said method modifies at least either a time-transitional increase property in the operating pressure for the hydraulic clutch to be engaged at the time of speed-changing or a time-transitional decrease property in the operating pressure for the hydraulic clutch to be disengaged at the time of speed-changing depending on whether or not said governor mechanism detects an engine load equal to or higher than a specific level.
 18. The method of performing hydraulic-pressure control according to claim 1, wherein the plurality of hydraulic clutches are classified and allocated in a plurality of hydraulic type speed change units connected in tandem, and the hydraulic clutches are alternatively engaged in each of said hydraulic type speed change units to thereby form one speed-change step.
 19. The method of performing hydraulic-pressure control according to claim 18, wherein, at the time of speed-changing operation, at least either a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch or a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary corresponding to the number of the clutches to be engaged/disengaged in the entirety of said speed change mechanism at the time of speed-changing.
 20. The method of performing hydraulic-pressure control according to claim 19, wherein, when the time difference is controlled to vary, the time difference is set longer in proportion to the increase in the number of the clutches to be engaged/disengaged.
 21. The method of performing hydraulic-pressure control according to claim 19, wherein, when the time-transitional decrease property is controlled to vary, the time-transitional decrease property is set to be reduced slower in proportion to the increase in the number of the clutches to be engaged/disengaged.
 22. The method of performing hydraulic-pressure control according to claim 19, wherein, during speed-changing, regardless of variations in the time difference or the time-transitional decrease property, an operating-hydraulic-pressure in the disengagement-objective clutch starts decreasing after a piston-holding pressure arises in the engagement-objective clutch by filling in a fluid chamber of the engagement-objective clutch.
 23. The method of performing hydraulic-pressure control according to claim 19, wherein, regardless of variations in the number of the clutches to be engaged/disengaged, the time-transitional pressure region where the engagement-objective clutch and the disengagement-objective clutch commonly slip at the time of speed-changing operation is maintained substantially to be constant.
 24. The method of performing hydraulic-pressure control according to claim 18, wherein pressure-detecting means is provided to detect an operating hydraulic pressure for each of the hydraulic clutches, and when two or more units of said pressure-detecting means detects a hydraulic pressure higher than a specific pressure value in one of said plurality of hydraulic type speed change units, one of two hydraulic-pressure control steps is performed, one hydraulic-pressure control step being performed to engage only those of the hydraulic clutches which have immediate-previously been disengaged, and the other one hydraulic-pressure control step being performed to disengage all the hydraulic clutches.
 25. The method of performing hydraulic-pressure control according to claim 24, switches being provided as said pressure-detecting means, wherein each of said switches turns ON or OFF with respect to the border of the specific pressure value to thereby determine whether the operating hydraulic pressure of corresponding one of the hydraulic clutches is higher or lower than the specific value. 